The implicit casting rule for `TO_BASE64` is not compatible with MySQL

## 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 TO_BASE64(1E+30);
+------------------+
| TO_BASE64(1E+30) |
+------------------+
| MWUzMA== |
+------------------+
1 row in set (0.02 sec)
```

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

```
mysql> SELECT TO_BASE64(1E+30);
+----------------------------------------------+
| TO_BASE64(1E+30) |
+----------------------------------------------+
| MTAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMA== |
+----------------------------------------------+
1 row in set (0.00 sec)
```

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