## 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.