- Corrected a wrong result that was recorded by the MySQL fix for BUG#39069.

- Removed Item_func_isnull::cached_value and all the logic around this custom-made
  caching of the NULL result because MWL#89 optimizes subqueries before the outer
  query is being executed, and this cache cannot be made easily to work for all
  kinds of Items (specifically Item_sum_sum, but others too).