The problem is in the Item_func_isnull::update_used_tables() function,
bracket is at the wrong place. Because of that isnull item erroneously
is treated as const item. The fix is to set brackets in the right place.


mysql-test/r/func_isnull.result:
  test case
mysql-test/t/func_isnull.test:
  test case
sql/item_cmpfunc.h:
  set brackets in the right place.