UNION converts NULL returned by AES_ENCRYPT with an invalid mode to an empty string in non-strict SQL mode

In non-strict SQL mode, AES_ENCRYPT() with an invalid encryption mode returns NULL in a SELECT DISTINCT query, but an equivalent UNION query returns an empty binary string.

The difference reproduces using the native mariadb command-line client. No tables are required.

Reproducer:

SET SESSION sql_mode = '';

SELECT DISTINCT AES_ENCRYPT('x', 'k', '', 'hkdf') AS col1;

SELECT AES_ENCRYPT('x', 'k', '', 'hkdf') AS col1

UNION

SELECT AES_ENCRYPT('x', 'k', '', 'hkdf') AS col1;

Actual result on 11.8.9-MariaDB:

The SELECT DISTINCT query returns one row containing NULL:

col1

NULL

The UNION query returns one row containing an empty binary string instead of NULL.

In MariaDB, the fourth argument of AES_ENCRYPT() specifies the encryption mode. Although 'hkdf' is not a valid encryption mode, using the same fixed arguments in equivalent SELECT DISTINCT and UNION queries should not change NULL into an empty string.