Fix incorrect default values of tf.sparse.to_dense
Fix incorrect default values of tf.sparse.to_dense

This fix tries to address the issue where tf.sparse.to_dense
without specifying default value explicitly leads to TypeError:
```
import tensorflow as tf
sample_string = tf.sparse.SparseTensor(indices=[[0, 0], [1, 2]], values=['a', 'b'], dense_shape=[3, 4])
tf.sparse.to_dense( sample_string )
...
TypeError: Expected string passed to parameter 'default_value' of op 'SparseToDense', got 0 of type 'int' instead. Error: Expected string, got 0 of type 'int' instead.
```

The issue was that tf.sparse.to_dense use 0 as the default value
which does not work well with string.

This fix changes from `default_value=0` -> `default_value=None`
and use zeros instead.

It consists of an API change though the change is backward compatible.

This fix fixes 30750

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

tensorflow/tensorflow,aldian/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-experimental_link_static_libraries_once,renyi533/tensorflow,jhseu/tensorflow,paolodedios/tensorflow,ppwwyyxx/tensorflow,tensorflow/tensorflow-pywrap_saved_model,petewarden/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-pywrap_tf_optimizer,jhseu/tensorflow,annarev/tensorflow,aam-at/tensorflow,chemelnucfin/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,arborh/tensorflow,Intel-Corporation/tensorflow,annarev/tensorflow,aam-at/tensorflow,sarvex/tensorflow,aam-at/tensorflow,davidzchen/tensorflow,sarvex/tensorflow,Intel-Corporation/tensorflow,xzturn/tensorflow,renyi533/tensorflow,annarev/tensorflow,Intel-tensorflow/tensorflow,freedomtan/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,DavidNorman/tensorflow,tensorflow/tensorflow,Intel-Corporation/tensorflow,karllessard/tensorflow,karllessard/tensorflow,annarev/tensorflow,frreiss/tensorflow-fred,DavidNorman/tensorflow,ppwwyyxx/tensorflow,chemelnucfin/tensorflow,chemelnucfin/tensorflow,annarev/tensorflow,Intel-tensorflow/tensorflow,chemelnucfin/tensorflow,aldian/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_saved_model,Intel-tensorflow/tensorflow,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_saved_model,petewarden/tensorflow,paolodedios/tensorflow,karllessard/tensorflow,davidzchen/tensorflow,yongtang/tensorflow,xzturn/tensorflow,yongtang/tensorflow,ppwwyyxx/tensorflow,gautam1858/tensorflow,DavidNorman/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,aam-at/tensorflow,renyi533/tensorflow,gunan/tensorflow,DavidNorman/tensorflow,yongtang/tensorflow,tensorflow/tensorflow,gunan/tensorflow,chemelnucfin/tensorflow,arborh/tensorflow,frreiss/tensorflow-fred,annarev/tensorflow,freedomtan/tensorflow,ppwwyyxx/tensorflow,freedomtan/tensorflow,freedomtan/tensorflow,renyi533/tensorflow,yongtang/tensorflow,jhseu/tensorflow,chemelnucfin/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,DavidNorman/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,ppwwyyxx/tensorflow,gunan/tensorflow,arborh/tensorflow,xzturn/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow,ppwwyyxx/tensorflow,Intel-tensorflow/tensorflow,Intel-Corporation/tensorflow,davidzchen/tensorflow,sarvex/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,Intel-Corporation/tensorflow,renyi533/tensorflow,arborh/tensorflow,freedomtan/tensorflow,jhseu/tensorflow,renyi533/tensorflow,annarev/tensorflow,petewarden/tensorflow,sarvex/tensorflow,annarev/tensorflow,aldian/tensorflow,sarvex/tensorflow,petewarden/tensorflow,DavidNorman/tensorflow,xzturn/tensorflow,xzturn/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,Intel-tensorflow/tensorflow,frreiss/tensorflow-fred,aam-at/tensorflow,aldian/tensorflow,jhseu/tensorflow,karllessard/tensorflow,tensorflow/tensorflow,gautam1858/tensorflow,cxxgtxy/tensorflow,gautam1858/tensorflow,chemelnucfin/tensorflow,cxxgtxy/tensorflow,aam-at/tensorflow,aldian/tensorflow,paolodedios/tensorflow,arborh/tensorflow,tensorflow/tensorflow,paolodedios/tensorflow,paolodedios/tensorflow,arborh/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,freedomtan/tensorflow,frreiss/tensorflow-fred,aldian/tensorflow,arborh/tensorflow,annarev/tensorflow,yongtang/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow,petewarden/tensorflow,adit-chandra/tensorflow,davidzchen/tensorflow,ppwwyyxx/tensorflow,aam-at/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,aam-at/tensorflow,chemelnucfin/tensorflow,davidzchen/tensorflow,gautam1858/tensorflow,xzturn/tensorflow,adit-chandra/tensorflow,chemelnucfin/tensorflow,aam-at/tensorflow,frreiss/tensorflow-fred,karllessard/tensorflow,gunan/tensorflow,tensorflow/tensorflow,petewarden/tensorflow,jhseu/tensorflow,cxxgtxy/tensorflow,annarev/tensorflow,jhseu/tensorflow,gautam1858/tensorflow,freedomtan/tensorflow,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_saved_model,cxxgtxy/tensorflow,petewarden/tensorflow,jhseu/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,jhseu/tensorflow,chemelnucfin/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,DavidNorman/tensorflow,xzturn/tensorflow,arborh/tensorflow,freedomtan/tensorflow,chemelnucfin/tensorflow,aldian/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,aldian/tensorflow,Intel-tensorflow/tensorflow,adit-chandra/tensorflow,davidzchen/tensorflow,adit-chandra/tensorflow,xzturn/tensorflow,chemelnucfin/tensorflow,yongtang/tensorflow,karllessard/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gunan/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,Intel-tensorflow/tensorflow,paolodedios/tensorflow,ppwwyyxx/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,freedomtan/tensorflow,gautam1858/tensorflow,renyi533/tensorflow,DavidNorman/tensorflow,renyi533/tensorflow,ppwwyyxx/tensorflow,Intel-Corporation/tensorflow,sarvex/tensorflow,adit-chandra/tensorflow,jhseu/tensorflow,Intel-tensorflow/tensorflow,arborh/tensorflow,ppwwyyxx/tensorflow,gunan/tensorflow,jhseu/tensorflow,xzturn/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,paolodedios/tensorflow,aam-at/tensorflow,cxxgtxy/tensorflow,DavidNorman/tensorflow,freedomtan/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,gunan/tensorflow,tensorflow/tensorflow,cxxgtxy/tensorflow,renyi533/tensorflow,adit-chandra/tensorflow,annarev/tensorflow,frreiss/tensorflow-fred,sarvex/tensorflow,gunan/tensorflow,freedomtan/tensorflow,ppwwyyxx/tensorflow,Intel-tensorflow/tensorflow,paolodedios/tensorflow,karllessard/tensorflow,renyi533/tensorflow,arborh/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-pywrap_saved_model,cxxgtxy/tensorflow,DavidNorman/tensorflow,gunan/tensorflow,petewarden/tensorflow,paolodedios/tensorflow,Intel-tensorflow/tensorflow,jhseu/tensorflow,davidzchen/tensorflow,yongtang/tensorflow,davidzchen/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_tf_optimizer,adit-chandra/tensorflow,renyi533/tensorflow,gautam1858/tensorflow,arborh/tensorflow,karllessard/tensorflow,adit-chandra/tensorflow,petewarden/tensorflow,aam-at/tensorflow,adit-chandra/tensorflow,xzturn/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_saved_model,xzturn/tensorflow,davidzchen/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,DavidNorman/tensorflow,gunan/tensorflow,renyi533/tensorflow,freedomtan/tensorflow,davidzchen/tensorflow,xzturn/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,yongtang/tensorflow,gunan/tensorflow,DavidNorman/tensorflow,frreiss/tensorflow-fred,adit-chandra/tensorflow,karllessard/tensorflow,cxxgtxy/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,sarvex/tensorflow,adit-chandra/tensorflow,arborh/tensorflow,yongtang/tensorflow,ppwwyyxx/tensorflow,gunan/tensorflow,petewarden/tensorflow,aam-at/tensorflow,Intel-tensorflow/tensorflow,paolodedios/tensorflow