Bug #121147 QUOTE() incorrectly treated as null-rejecting in outer joins

Description:
Per the manual / implementation notes, `QUOTE(NULL)` returns the **string** `'NULL'` (four letters), not SQL NULL.  
Therefore predicates such as `QUOTE(inner_col) IS NOT NULL` or `QUOTE(inner_col) > ''` are **true** for NULL-complemented outer-join rows and must not be used to convert a LEFT JOIN into an INNER JOIN.

On MySQL 9.7.1, `QUOTE(inner_col)` in WHERE is still treated as null-rejecting:

- `EXPLAIN` shows an **Inner hash join** instead of a left join.
- `COUNT(*)` with `WHERE QUOTE(t2.b) IS NOT NULL` returns **1**.
- The same predicate evaluated via `QUOTE(CASE WHEN FALSE THEN t2.b ELSE t2.b END)` returns **2** (correct).
- Filtering the predicate in an outer subquery also returns **2**.

How to repeat:
```sql
CREATE TABLE t1 (a INT, b INT);
INSERT INTO t1 VALUES (1,1),(2,3);

-- Semantics: QUOTE(NULL) is not SQL NULL
SELECT QUOTE(NULL) IS NOT NULL;           -- 1
SELECT CONCAT('X', QUOTE(NULL));          -- 'XNULL'

-- SELECT-list: NULL-extended row has QUOTE(...) IS NOT NULL = 1
SELECT t1.a, t2.b, QUOTE(t2.b) IS NOT NULL AS p, QUOTE(t2.b) AS q
FROM t1 LEFT JOIN t1 t2 ON t1.a = t2.b;
-- 1 | 1    | 1 | '1'
-- 2 | NULL | 1 | NULL   << string NULL (IS NOT NULL = 1), not SQL NULL

-- WHERE (wrong)
SELECT COUNT(*) AS c_where
FROM t1 LEFT JOIN t1 t2 ON t1.a = t2.b
WHERE QUOTE(t2.b) IS NOT NULL;
-- Actual: 1
-- Expected: 2

-- CASE wrapper (correct) — same pattern as Bug #112397
SELECT COUNT(*) AS c_case
FROM t1 LEFT JOIN t1 t2 ON t1.a = t2.b
WHERE QUOTE(CASE WHEN FALSE THEN t2.b ELSE t2.b END) IS NOT NULL;
-- Actual: 2
-- Expected: 2

-- Outer filter oracle (correct)
SELECT COUNT(*) AS c_subq FROM (
  SELECT t1.a, QUOTE(t2.b) IS NOT NULL AS p
  FROM t1 LEFT JOIN t1 t2 ON t1.a = t2.b
) s WHERE p;
-- Actual: 2
-- Expected: 2

EXPLAIN FORMAT=TREE
SELECT * FROM t1 LEFT JOIN t1 t2 ON t1.a = t2.b
WHERE QUOTE(t2.b) IS NOT NULL;
-- Shows: Inner hash join ... (should remain Left join)

DROP TABLE t1;
```

Also reproduces with:

```sql
WHERE QUOTE(t2.b) > '';
-- Actual COUNT: 1; Expected: 2

Thank you for the bug report.
Verified as described.