`REGEXP_REPLACE` is not compatible with MySQL 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 REGEXP_REPLACE('', '^$', 'empty');
+-----------------------------------+
| REGEXP_REPLACE('', '^$', 'empty') |
+-----------------------------------+
| |
+-----------------------------------+
1 row in set (0.02 sec)
```

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

```
mysql> SELECT REGEXP_REPLACE('', '^$', 'empty');
+-----------------------------------+
| REGEXP_REPLACE('', '^$', 'empty') |
+-----------------------------------+
| empty |
+-----------------------------------+
1 row in set (0.00 sec)
```

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