If condition in UnionAllRecordBatch which was changed to correct one in
DRILL-5419 but this change caused the regression (DRILL-5521).
The root cause is not in change itself but rather in missing indication of child column in map types when it is null. There is a big project upcoming re-working json reader implementation, including map types (DRILL-4824) which will fix the problem.
So far reverting change to previous one so users won't see the regression till DRILL-4824 is merged.