MariaDB [test]> SELECT JSON_MERGE('[]', '{"c":"d"}');

+-------------------------------+

| JSON_MERGE('[]', '{"c":"d"}') |

+-------------------------------+

| NULL                          |

+-------------------------------+

1 row in set, 1 warning (0.00 sec)

MariaDB [test]> SHOW WARNINGS;

+---------+------+--------------------------------------------------------------------------------+

| Level   | Code | Message                                                                        |

+---------+------+--------------------------------------------------------------------------------+

| Warning | 4038 | Syntax error in JSON text in argument 1 to function 'json_merge' at position 2 |

+---------+------+--------------------------------------------------------------------------------+

1 row in set (0.00 sec)

MariaDB [test]> SELECT JSON_VALID('[]');

+------------------+

| JSON_VALID('[]') |

+------------------+

|                1 |

+------------------+

1 row in set (0.00 sec)