Inconsistent warnings when converting string to decimal

mysql> select cast('' as decimal(30,1)), cast(concat('') as decimal(30,1)); show warnings;

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

| cast('' as decimal(30,1)) | cast(concat('') as decimal(30,1)) |

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

|                       0.0 |                               0.0 |

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

1 row in set, 2 warnings (0.00 sec)

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

| Level   | Code | Message                                                 |

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

| Warning | 1292 | Truncated incorrect DECIMAL value: ''                   |

| Warning | 1918 | Encountered illegal value '' when converting to DECIMAL |

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

2 rows in set (0.00 sec)