MDEV-33665: MSAN failure due to uninitialized Item_func::not_null_tables_cache

eliminate_item_equal() uses quick_fix_field() for Item objects it creates.
It computes some of their attributes on its own (see update_used_tables()
call) but it doesn't update not_null_tables_cache.

Recompute not_null_tables_cache also. Not computing it is currently
harmless, except for producing MSAN error when some other code
propagates the wrong value of not_null_tables_cache to other item.

sql/sql_select.cc

@@ -16209,6 +16209,7 @@ Item *eliminate_item_equal(THD *thd, COND *cond, COND_EQUAL *upper_levels,
 
       if (!eq_item || eq_item->set_cmp_func(thd))
         return 0;
+      eq_item->eval_not_null_tables(0);
       eq_item->quick_fix_field();
     }
     current_sjm= field_sjm;
@@ -16266,6 +16267,7 @@ Item *eliminate_item_equal(THD *thd, COND *cond, COND_EQUAL *upper_levels,
   {
     res->quick_fix_field();
     res->update_used_tables();
+    res->eval_not_null_tables(0);
   }
 
   return res;