CONVERT produces a warning and result depends on strict mode

MariaDB [test]> set sql_mode='';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select convert(_big5 0xC84041 using ucs2);

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

| convert(_big5 0xC84041 using ucs2) |

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

| ?A                                 |

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

1 row in set, 1 warning (0.00 sec)

MariaDB [test]> show warnings;

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

| Level   | Code | Message                                          |

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

| Warning | 1977 | Cannot convert 'big5' character 0xC840 to 'ucs2' |

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

1 row in set (0.00 sec)

MariaDB [test]> set sql_mode='STRICT_ALL_TABLES';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select convert(_big5 0xC84041 using ucs2);

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

| convert(_big5 0xC84041 using ucs2) |

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

| NULL                               |

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

1 row in set, 1 warning (0.01 sec)

MariaDB [test]> show warnings;

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

| Level   | Code | Message                                          |

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

| Warning | 1977 | Cannot convert 'big5' character 0xC840 to 'ucs2' |

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

1 row in set (0.00 sec)