`TIME` returns zero time instead of NULL for invalid time

## 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)

Start by reproducing the reported SELECT TIME('') and SELECT TIME('NOT_A_TIME') cases and compare the actual and expected results. Trace the TIME function implementation and its existing tests; done means invalid inputs return NULL with a warning instead of 00:00:00.

Written by the indexing model from the issue text.