TIMESTAMP should return NULL for zero time

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

mysql> SELECT TIMESTAMP('0000-00-00 00:00:00');
+----------------------------------+
| TIMESTAMP('0000-00-00 00:00:00') |
+----------------------------------+
| NULL                             |
+----------------------------------+
1 row in set, 1 warning (0.02 sec)

3. What did you see instead (Required)

mysql> SELECT TIMESTAMP('0000-00-00 00:00:00');
+----------------------------------+
| TIMESTAMP('0000-00-00 00:00:00') |
+----------------------------------+
| 0000-00-00 00:00:00              |
+----------------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[zanmato1984]

/severity moderate