`INET6_NTOA` should return `null` for invalid address

## 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 INET6_NTOA(1234);
+------------------+
| INET6_NTOA(1234) |
+------------------+
| NULL |
+------------------+
1 row in set, 1 warning (0.03 sec)

mysql> SELECT INET6_NTOA('abcdefghijklmnop');
+--------------------------------+
| INET6_NTOA('abcdefghijklmnop') |
+--------------------------------+
| NULL |
+--------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------+
| Warning | 1411 | Incorrect string value: ''abcdefghijklmnop'' for function inet6_ntoa |
+---------+------+----------------------------------------------------------------------+
1 row in set (0.01 sec)
```

### 3. What did you see instead (Required)

```

mysql> SELECT INET6_NTOA(1234);
+------------------+
| INET6_NTOA(1234) |
+------------------+
| 49.50.51.52 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT INET6_NTOA('abcdefghijklmnop');
+-----------------------------------------+
| INET6_NTOA('abcdefghijklmnop') |
+-----------------------------------------+
| 6162:6364:6566:6768:696a:6b6c:6d6e:6f70 |
+-----------------------------------------+
1 row in set (0.00 sec)
```

### 4. What is your TiDB version? (Required)