Wrong COUNT(*) result in correlated scalar subquery with multiple NULLs in a nullable UNIQUE key

A correlated scalar subquery with COUNT returns an incorrect result
when a nullable UNIQUE column contains multiple NULL values.

Tested on MariaDB 12.3.3 with the default optimizer_switch.
The issue reproduces with both MyISAM and InnoDB.

Minimal reproducer:

CREATE TABLE n(c INT UNIQUE) ENGINE=MyISAM;
INSERT INTO n VALUES (NULL), (NULL);

SELECT (SELECT COUNT FROM n z WHERE z.c <=> x.k) AS scalar_count
FROM (SELECT NULL AS k) x;

SELECT COUNT AS direct_count
FROM n WHERE c IS NULL;

Actual result:

scalar_count
------------
1

direct_count
------------
2

Expected result:

scalar_count
------------
2

The nullable UNIQUE column permits both rows with c = NULL.

In the correlated scalar subquery, x.k is NULL. Since <=> is
NULL-safe equality, both rows where z.c is NULL satisfy:

z.c <=> x.k

Therefore, COUNT should return 2.

The direct query:

SELECT COUNT FROM n WHERE c IS NULL;

correctly returns 2, while the correlated scalar form incorrectly
returns 1.

The same wrong result is reproduced when the table uses InnoDB
instead of MyISAM.