Allow creating nullptr AttrHeaderBase to support TypedAttrBase::dyn_cast. TypedAttrBase::dyn_cast should return a nullptr if type mismatch.
Allow creating nullptr AttrHeaderBase to support TypedAttrBase::dyn_cast. TypedAttrBase::dyn_cast should return a nullptr if type mismatch.

PiperOrigin-RevId: 333419997
Change-Id: Ib3e9d25b0cf95dbb37117f52ded3ec8478f4131f

annarev/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,davidzchen/tensorflow,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_saved_model,cxxgtxy/tensorflow,karllessard/tensorflow,sarvex/tensorflow,petewarden/tensorflow,Intel-tensorflow/tensorflow,yongtang/tensorflow,annarev/tensorflow,petewarden/tensorflow,karllessard/tensorflow,annarev/tensorflow,tensorflow/tensorflow,petewarden/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,frreiss/tensorflow-fred,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-experimental_link_static_libraries_once,annarev/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,sarvex/tensorflow,davidzchen/tensorflow,sarvex/tensorflow,freedomtan/tensorflow,Intel-tensorflow/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-Corporation/tensorflow,aam-at/tensorflow,annarev/tensorflow,aam-at/tensorflow,yongtang/tensorflow,aam-at/tensorflow,freedomtan/tensorflow,freedomtan/tensorflow,freedomtan/tensorflow,karllessard/tensorflow,aam-at/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,frreiss/tensorflow-fred,annarev/tensorflow,sarvex/tensorflow,gautam1858/tensorflow,aam-at/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,paolodedios/tensorflow,frreiss/tensorflow-fred,aam-at/tensorflow,aam-at/tensorflow,paolodedios/tensorflow,petewarden/tensorflow,annarev/tensorflow,petewarden/tensorflow,paolodedios/tensorflow,frreiss/tensorflow-fred,freedomtan/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,aam-at/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,frreiss/tensorflow-fred,tensorflow/tensorflow,aam-at/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,davidzchen/tensorflow,Intel-tensorflow/tensorflow,aam-at/tensorflow,tensorflow/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,cxxgtxy/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,davidzchen/tensorflow,davidzchen/tensorflow,Intel-Corporation/tensorflow,gautam1858/tensorflow,Intel-tensorflow/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,sarvex/tensorflow,paolodedios/tensorflow,paolodedios/tensorflow,frreiss/tensorflow-fred,freedomtan/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow,freedomtan/tensorflow,karllessard/tensorflow,davidzchen/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,yongtang/tensorflow,Intel-Corporation/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_tf_optimizer,karllessard/tensorflow,davidzchen/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,freedomtan/tensorflow,gautam1858/tensorflow,karllessard/tensorflow,karllessard/tensorflow,paolodedios/tensorflow,karllessard/tensorflow,Intel-Corporation/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow,cxxgtxy/tensorflow,paolodedios/tensorflow,Intel-tensorflow/tensorflow,cxxgtxy/tensorflow,cxxgtxy/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow,karllessard/tensorflow,tensorflow/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,petewarden/tensorflow,davidzchen/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_tf_optimizer,frreiss/tensorflow-fred,petewarden/tensorflow,aam-at/tensorflow,Intel-tensorflow/tensorflow,aam-at/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_tf_optimizer,freedomtan/tensorflow,sarvex/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-pywrap_saved_model,annarev/tensorflow,tensorflow/tensorflow,davidzchen/tensorflow,freedomtan/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,freedomtan/tensorflow,freedomtan/tensorflow,yongtang/tensorflow,tensorflow/tensorflow,cxxgtxy/tensorflow,cxxgtxy/tensorflow,petewarden/tensorflow,petewarden/tensorflow,yongtang/tensorflow,karllessard/tensorflow,annarev/tensorflow,annarev/tensorflow,gautam1858/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,cxxgtxy/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,annar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