[SPARK-14202] [PYTHON] Use generator expression instead of list comp in python_full_outer_jo…

## What changes were proposed in this pull request?

This PR replaces list comprehension in python_full_outer_join.dispatch with a generator expression.

## How was this patch tested?

PySpark-Core, PySpark-MLlib test suites against Python 2.7, 3.5.

Author: zero323 <matthew.szymkiewicz@gmail.com>

Closes #11998 from zero323/pyspark-join-generator-expr.

python/pyspark/join.py

@@ -93,7 +93,7 @@ def python_full_outer_join(rdd, other, numPartitions):
             vbuf.append(None)
         if not wbuf:
             wbuf.append(None)
-        return [(v, w) for v in vbuf for w in wbuf]
+        return ((v, w) for v in vbuf for w in wbuf)
     return _do_python_join(rdd, other, numPartitions, dispatch)