Item_in_optimizer::is_null() evaluated "NULL IN (SELECT ...)" to NULL regardless of
whether subquery produced any records, this was a documented limitation.

The limitation has been removed (see bugs 8804, 24085, 24127) now
Item_in_optimizer::val_int() correctly handles all cases with NULLs. Make
Item_in_optimizer::is_null() invoke val_int() to return correct values for
"NULL IN (SELECT ...)".