When single-row subquery fails with "Subquery reutrns more than 1 row"
error, it will raise an error and return NULL.

On the other hand, Item_singlerow_subselect sets item->maybe_null=0
for table-less subqueries like "(SELECT not_null_value)"  (*)

This discrepancy (item with maybe_null=0 returning NULL) causes the
code in Type_handler_decimal_result::make_sort_key_part() to crash.

Fixed this by allowing inference (*) only when the subquery is NOT a
UNION.