Fix build issue while creating Python API

This fix tries to address the issue raised in 20233 where
build could fail with the following error:
```
"/var/tmp/portage/sci-libs/tensorflow-1.9.0_rc1/work/bazel-base-python3_6/execroot/org_tensorflow/bazel-out/host/bin/tensorflow/tools/api/generator/create_python_api.runfiles/org_tensorflow/tensorflow/tools/api/generator/create_python_api.py", line 182, in get_api_init_text
    package not in module.__name__):
TypeError: argument of type 'NoneType' is not iterable
Target //tensorflow/tools/pip_package:build_pip_package failed to build
INFO: Elapsed time: 3202.457s, Critical Path: 83.98s
INFO: 4901 processes, local.
FAILED: Build did NOT complete successfully
FAILED: Build did NOT complete successfully
```

This fix addresses the issue.

This fix fixes 20233.

NOTE: The patch is provided by @cjmayo

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>