The implicit casting rule for `WEIGHT_STRING` is not correct

## 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 WEIGHT_STRING(123);
+----------------------------------------+
| WEIGHT_STRING(123) |
+----------------------------------------+
| 0x800000000000007B |
+----------------------------------------+
1 row in set (0.01 sec)

mysql> SELECT WEIGHT_STRING(TRUE);
+------------------------------------------+
| WEIGHT_STRING(TRUE) |
+------------------------------------------+
| 0x8000000000000001 |
+------------------------------------------+
1 row in set (0.01 sec)

mysql> SELECT WEIGHT_STRING('a' COLLATE utf8mb4_bin);
+--------------------------------------------------------------------------------+
| WEIGHT_STRING('a' COLLATE utf8mb4_bin) |
+--------------------------------------------------------------------------------+
| 0x000061 |
+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

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

```
mysql> SELECT WEIGHT_STRING(123);
+----------------------------------------+
| WEIGHT_STRING(123) |
+----------------------------------------+
| NULL |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT WEIGHT_STRING(TRUE);
+------------------------------------------+
| WEIGHT_STRING(TRUE) |
+------------------------------------------+
| NULL |
+------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT WEIGHT_STRING('a' COLLATE utf8mb4_bin);
+--------------------------------------------------------------------------------+
| WEIGHT_STRING('a' COLLATE utf8mb4_bin) |
+--------------------------------------------------------------------------------+
| 0x61 |
+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

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