Added test case for bug #4256.
join_outer.result:
  Fixed bug #4256.
item_cmpfunc.h:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects.
  This inconsistency caused bug #4256.


sql/item_cmpfunc.h:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects.
  This inconsistency caused bug #4256.
mysql-test/r/join_outer.result:
  Fixed bug #4256.
mysql-test/r/null.result:
  Added test case for bug #4256.
mysql-test/t/null.test:
  Added test case for bug #4256.