`BIN` should return `NULL` for empty string

## 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 BIN('');
+---------+
| BIN('') |
+---------+
| NULL |
+---------+
1 row in set (0.01 sec)
```

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

```
mysql> SELECT BIN('');
+---------+
| BIN('') |
+---------+
| 0 |
+---------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: '' |
+---------+------+---------------------------------------+
1 row in set (0.01 sec)
```

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