[SPARK-16409][SQL] regexp_extract with optional groups causes NPE

## What changes were proposed in this pull request?

regexp_extract actually returns null when it shouldn't when a regex matches but the requested optional group did not. This makes it return an empty string, as apparently designed.

## How was this patch tested?

Additional unit test

Author: Sean Owen <sowen@cloudera.com>

Closes #14504 from srowen/SPARK-16409.