`TIME` returns zero time instead of NULL for invalid 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 TIME('');
+----------+
| TIME('') |
+----------+
| NULL     |
+----------+
1 row in set, 1 warning (0.02 sec)

mysql> SELECT TIME('NOT_A_TIME');
+--------------------+
| TIME('NOT_A_TIME') |
+--------------------+
| NULL               |
+--------------------+
1 row in set, 1 warning (0.00 sec)

3. What did you see instead (Required)

mysql> SELECT TIME('');
+----------+
| TIME('') |
+----------+
| 00:00:00 |
+----------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT TIME('NOT_A_TIME');
+--------------------+
| TIME('NOT_A_TIME') |
+--------------------+
| 00:00:00           |
+--------------------+
1 row in set, 1 warning (0.00 sec)

4. What is your TiDB version? (Required)

[zanmato1984]

/severity moderate