mysqltest: TRIM(NULL) Returns Empty String Instead of NULL

let $tt1 = $(trim(null));

-- echo trim(null) -> $tt1;           

worker[01] Using MTR_BUILD_THREAD 300, with reserved ports 19000..19029

trim(null) -> ;                

main.trim1                               [ pass ]    

MariaDB [(none)]> select trim(null);

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

| trim(null) |

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

| NULL       |

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

1 row in set (0.001 sec)