Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects (bug #1990).


sql/item_cmpfunc.h:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects (bug #1990).
mysql-test/t/null.test:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects (bug #1990).
mysql-test/r/null.result:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects (bug #1990).
mysql-test/r/join_outer.result:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects (bug #1990).