MDEV-14515 ifnull result depends on number of rows in joined table

Any expensive WHERE condition for a table-less query with
implicit aggregation was lost. As a result the used aggregate
functions were calculated over a non-empty set of rows even
in the case when the condition was false.

sql/opt_subselect.cc

@@ -5903,5 +5903,6 @@ bool JOIN::choose_tableless_subquery_plan()
       tmp_having= having;
     }
   }
+  exec_const_cond= conds;
   return FALSE;
 }