`GET_FORMAT` returns empty string instead of `NULL` for unknown format

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

Run `SELECT GET_FORMAT(DATE, 'FOO');`

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

```
mysql> SELECT GET_FORMAT(DATE, 'FOO');
+-------------------------+
| GET_FORMAT(DATE, 'FOO') |
+-------------------------+
| NULL |
+-------------------------+
1 row in set (0.01 sec)
```

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

```
mysql> SELECT GET_FORMAT(DATE, 'FOO');
+-------------------------+
| GET_FORMAT(DATE, 'FOO') |
+-------------------------+
| |
+-------------------------+
1 row in set (0.00 sec)

```

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