Inconsistent Behavior of GROUP BY ROLLUP in v17 vs master

[邱宇航]

I've noticed that two GROUP BY ROLLUP queries behave differently in v17
compared to master and REL_18_STABLE. The issue can be reproduced by
following SQL:

``` SQL
CREATE TABLE t(id int);

INSERT INTO t SELECT generate_series(1, 3);

-- Query 1
SELECT DISTINCT 'XXX'
FROM t
GROUP BY ROLLUP (id, 1);

-- Query 2
SELECT 'XXX'
FROM t
GROUP BY ROLLUP(id)
HAVING NOT (NULL IS NULL);
```

After some git bisect work, I traced the root cause:
- The first issue was introduced by commit f5050f79 (Mark expressions
nullable by grouping sets).
- The second issue stems from commit 67a54b9e (Allow pushdown of HAVING
clauses with grouping sets).

Best regards,
Yuhang Qiu