PASSWORD(NULL) returns empty string, should be NULL according to documentation

MariaDB [test]> select password(null);

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

| password(null) |

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

|                |

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

1 row in set (0.00 sec)

https://mariadb.com/kb/en/mariadb/password

Calculates and returns a hashed password string from the plaintext password str, or NULL if the argument was NULL.

Same in MySQL manual: http://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_password

MySQL 5.6/5.7 are also affected.