zeroDate behavior for date_add and timestampdiff is not consistent in tidb

Description

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

mysql> select timestampdiff(day,'0000-00-00 00:00:00','2020-01-01 00:00:00');
+----------------------------------------------------------------+
| timestampdiff(day,'0000-00-00 00:00:00','2020-01-01 00:00:00') |
+----------------------------------------------------------------+
|                                                           NULL |
+----------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select date_add('0000-00-00 00:00:00', interval 2020 year);
+-----------------------------------------------------+
| date_add('0000-00-00 00:00:00', interval 2020 year) |
+-----------------------------------------------------+
| 2019-11-30 00:00:00                                 |
+-----------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

2. What did you expect to see? timestampdiff and date_add should both return NULL or meaningful result. BTW in mysql both timestampdiff and date_add return NULL

3. What did you see instead?

mysql> select timestampdiff(day,'0000-00-00 00:00:00','2020-01-01 00:00:00');
+----------------------------------------------------------------+
| timestampdiff(day,'0000-00-00 00:00:00','2020-01-01 00:00:00') |
+----------------------------------------------------------------+
|                                                           NULL |
+----------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select date_add('0000-00-00 00:00:00', interval 2020 year);
+-----------------------------------------------------+
| date_add('0000-00-00 00:00:00', interval 2020 year) |
+-----------------------------------------------------+
| 2019-11-30 00:00:00                                 |
+-----------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

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

tidb master