Add `skip_empty` option to `tf.string_split` (#12125)

* Add `skip_empty` option to `tf.string_split`

This fix add `skip_empty` to `tf.string_split` so that
it is possible to have `tf.string_split` behaving similiarly
with python.

This fix fixes 12108.

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

* Update goldens

Run through:
```
bazel-bin/tensorflow/tools/api/tests/api_compatibility_test           --update_goldens True
```
to pass //tensorflow/tools/api/tests:api_compatibility_test

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