[SPARK-11060] [STREAMING] Fix some potential NPE in DStream transformation

This patch fixes:

1. Guard out against NPEs in `TransformedDStream` when parent DStream returns None instead of empty RDD.
2. Verify some input streams which will potentially return None.
3. Add unit test to verify the behavior when input stream returns None.

cc tdas , please help to review, thanks a lot :).

Author: jerryshao <sshao@hortonworks.com>

Closes #9070 from jerryshao/SPARK-11060.