ROLLUP marker NULL treated as data NULL in correlated subqueries

### Description When a correlated subquery references a column from an outer query that uses `GROUP BY ... WITH ROLLUP`, the super-aggregate (ROLLUP) row's NULL value in the GROUP BY column is incorrectly treated as a data NULL rather than a structural marker NULL. This causes the correlated subquery to return incorrect results on the ROLLUP row. The SQL standard requires that `NULL = NULL` evaluates to UNKNOWN (i.e., no match), so a correlated subquery on the ROLLUP row (where the correlation column is NULL) should return 0 / NULL / FALSE. However, MariaDB incorrectly returns a non-zero count or non-NULL value, indicating that the marker NULL is being matched against actual data. ### Minimal Reproduction ```sql CREATE TABLE t0 (id INT PRIMARY KEY, c0 INT); CREATE TABLE t1 (id INT PRIMARY KEY, c0 INT); INSERT INTO t0 VALUES (1, 10), (2, 20), (3, 30); INSERT INTO t1 VALUES (1, 10), (2, 20), (3, 20), (4, 30); SELECT t0.c0, (SELECT COUNT(*) FROM t1 WHERE t1.c0 = t0.c0) AS cnt FROM t0 GROUP BY t0.c0 WITH ROLLUP; ``` ### Expected Result The ROLLUP super-aggregate row (c0=NULL) should have `cnt=0`, because `t1.c0 = NULL` evaluates to UNKNOWN and matches no rows: ``` c0 cnt 10 1 20 2 30 1 NULL 0 ← correct ``` ### Actual Result MariaDB returns `cnt=1` on the ROLLUP row, indicating the marker NULL is incorrectly matched against data: ``` c0 cnt 10 1 20 2 30 1 NULL 1 ← WRONG ```