Python typehints: spurious warnings for Generator None args

GeneratorHint outputs warnings if the 2 last args of typing.Generator are not None.
1. Compare to type(None) instead of None, since that's what the code converts those to. (assert on logging output)
2. (fixed) Bug: comparing send_type in both cases.
https://github.com/apache/beam/blob/06fa1cc2a92a6f4f0e05f162ee2eb78b072920d9/sdks/python/apache_beam/typehints/typehints.py#L1088-L1091