[SPARK-19882][SQL] Pivot with null as a distinct pivot value throws NPE

## What changes were proposed in this pull request?

Allows null values of the pivot column to be included in the pivot values list without throwing NPE

Note this PR was made as an alternative to #17224 but preserves the two phase aggregate operation that is needed for good performance.

## How was this patch tested?

Additional unit test

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #17226 from aray/pivot-null.