When executing GROUP BY NULL, Mariadb returns unexpected results

CREATE TABLE t0 (

  c0 double unsigned zerofill NULL

);

CREATE TABLE t0 (

  c0 double unsigned NULL

);

INSERT INTO t0 VALUES (0.1251773127435537), (NULL), (0), (0.6665588482250941);

CREATE TABLE t1(c0 BOOL);

INSERT INTO t1 VALUES (NULL), (NULL), (0), (0);

SELECT t0.c0 FROM t0 NATURAL RIGHT JOIN t1 GROUP BY NULL;

+------+

| c0   |

+------+

| NULL |

+------+

MariaDB> SELECT t0.c0 FROM  t0 NATURAL RIGHT JOIN t1 GROUP BY NULL;

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

| c0                     |

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

| 0000000000000000000000 |

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

1 row in set (0.000 sec)