[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.

python/pyspark/sql/dataframe.py

@@ -566,8 +566,7 @@ class DataFrame(object):
 
         if on is None or len(on) == 0:
             jdf = self._jdf.join(other._jdf)
-
-        if isinstance(on[0], basestring):
+        elif isinstance(on[0], basestring):
             jdf = self._jdf.join(other._jdf, self._jseq(on))
         else:
             assert isinstance(on[0], Column), "on should be Column or list of Column"