CONVERT_TZ should return NULL if its arguments are invalid.

[SunRunAway]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

SELECT CONVERT_TZ( '2004-01-01 12:00:00', '', '');
SELECT CONVERT_TZ( '2004-01-01 12:00:00', 'a', '');
SELECT CONVERT_TZ( '2004-01-01 12:00:00', 0, '');

2. What did you expect to see?

Due to https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_convert-tz,

This function returns NULL if the arguments are invalid.

mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', '', '');
+--------------------------------------------+
| CONVERT_TZ( '2004-01-01 12:00:00', '', '') |
+--------------------------------------------+
| NULL                                       |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', 'a', '');
+---------------------------------------------+
| CONVERT_TZ( '2004-01-01 12:00:00', 'a', '') |
+---------------------------------------------+
| NULL                                        |
+---------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', 0, '');
+-------------------------------------------+
| CONVERT_TZ( '2004-01-01 12:00:00', 0, '') |
+-------------------------------------------+
| NULL                                      |
+-------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', '', '');
+--------------------------------------------+
| CONVERT_TZ( '2004-01-01 12:00:00', '', '') |
+--------------------------------------------+
| 2004-01-01 12:00:00                        |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', 'a', '');
ERROR 1105 (HY000): unknown time zone a
mysql> SELECT CONVERT_TZ( '2004-01-01 12:00:00', 0, '');
ERROR 1105 (HY000): unknown time zone 0

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

[tcmichael]

/assign