When using --fr (regex field selector) with stats1 -a null_count, void
(empty) field values were unconditionally skipped in
ingestWithValueFieldRegexes, causing null_count to always report 0.
The non-regex path (ingestWithoutValueFieldRegexes) already had a special
case that allows void values through for null_count accumulators. This
commit adds the same exception to the regex path.
Fixes#1639
Co-authored-by: cobyfrombrooklyn-bot <cobyfrombrooklyn-bot@users.noreply.github.com>
