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

```