Fix TypeError while importing tensorflow caused by site.USER_SITE is set to None
Fix TypeError while importing tensorflow caused by site.USER_SITE is set to None

This PR tries to address the issue raised in 34643 where the following may cause
import error:
```
$ python3 -S
>>> import sys
>>> sys.path=['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/usr/local/lib/python3.6/dist-packages', '/usr/lib/python3/dist-packages']
>>> import tensorflow
2020-04-18 02:53:44.825113: W tensorflow/stream_executor/platform/default/dso_loader.cc:55] Could not load dynamic library 'libcuda.so.1'; dlerror: libcuda.so.1: cannot open shared object file: No such file or directory
2020-04-18 02:53:44.825163: E tensorflow/stream_executor/cuda/cuda_driver.cc:313] failed call to cuInit: UNKNOWN ERROR (303)
2020-04-18 02:53:44.825191: I tensorflow/stream_executor/cuda/cuda_diagnostics.cc:156] kernel driver does not appear to be running on this host (7d29dc61d5ee): /proc/driver/nvidia/version does not exist
2020-04-18 02:53:44.825441: I tensorflow/core/platform/cpu_feature_guard.cc:143] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
2020-04-18 02:53:44.849778: I tensorflow/core/platform/profile_utils/cpu_utils.cc:102] CPU Frequency: 2199995000 Hz
2020-04-18 02:53:44.850142: I tensorflow/compiler/xla/service/service.cc:168] XLA service 0x7fc308000b20 initialized for platform Host (this does not guarantee that XLA will be used). Devices:
2020-04-18 02:53:44.850178: I tensorflow/compiler/xla/service/service.cc:176]   StreamExecutor device (0): Host, Default Version
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/tensorflow/__init__.py", line 417, in <module>
    if _running_from_pip_package():
  File "/usr/local/lib/python3.6/dist-packages/tensorflow/__init__.py", line 415, in _running_from_pip_package
    _current_file_location.startswith(dir_) for dir_ in _site_packages_dirs)
  File "/usr/local/lib/python3.6/dist-packages/tensorflow/__init__.py", line 415, in <genexpr>
    _current_file_location.startswith(dir_) for dir_ in _site_packages_dirs)
TypeError: startswith first arg must be str or a tuple of str, not NoneType
```

The reason was that `site.USER_SITE` could be None.

This PR removes None with:
```
_site_packages_dirs += [] if _site.USER_SITE is None else [_site.USER_SITE]
```

This PR fixes 34643

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

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