[SPARK-9726] [PYTHON] PySpark DF join no longer accepts on=None

rxin

First pull request for Spark so let me know if I am missing anything
The contribution is my original work and I license the work to the project under the project's open source license.

Author: Brennan Ashton <bashton@brennanashton.com>

Closes #8016 from btashton/patch-1.