Objects of the classes Item_func_is_not_null_test and Item_func_trig_cond
must be transparent for the method Item::split_sum_func2 as these classes
are pure helpers. It means that the method Item::split_sum_func2 should
look at those objects as at pure wrappers.


mysql-test/r/subselect3.result:
  Added a test case for bug #24420.
mysql-test/t/subselect3.test:
  Added a test case for bug #24420.