- "Early NULLs filtering" optimization used to "peel off" Item_ref and
  Item_direct_ref wrappers from an outside column reference before
  adding "outer_table_col IS NOT NULL" into JOIN::outer_ref_cond.
- When this happened in a subquery that was evaluated in a post-GROUP-BY
  context, attempt to evaluate JOIN::outer_ref_cond would fetch an
  incorrect value of outer_table_col.