Unexpected results when using `LEFT JOIN` and `UNION`

[suyZhong]

Consider the test case below. It is unexpected that the third query return 1 'a', as the first query return 1 NULL, and the second query also return 1 NULL.

DROP TABLE t0;
DROP TABLE t1;

CREATE TABLE t0(c0 INT);
CREATE TABLE t1(c0 VARCHAR(500));
INSERT INTO t0 (c0) VALUES (1);
INSERT INTO t1 (c0) VALUES ('a');

SELECT * FROM t0 LEFT  JOIN t1 ON (NULL IN ('')); -- 1 NULL
SELECT * FROM t0 LEFT  JOIN t1 ON (NULL IN ('')) WHERE 1; -- 1 NULL
SELECT * FROM t0 LEFT  JOIN t1 ON (NULL IN ('')) WHERE 1 UNION ALL SELECT * FROM t0 LEFT  JOIN t1 ON (NULL IN ('')) WHERE (NOT 1);
-- Expected: 1 NULL
-- Actual: 1 'a'

Here's the version: Version 7.2.13-dev.3239-pthreads as of Mar 13 2024 (bb53b41)