You can't select NULL in QV by default.
Besides using statements like NullAsValue or a isNull check on field values in your script (please refer to the script), which probably doesn't help here, I would suggest that you use
as expression, then make the expression searchable and use an empty search string. This should filter all empty Region (I assume you don't have empty, but not NULL, original Region values, or you might want to check them as well).
Hope this helps,