Result of UNIX_TIMESTAMP(1) is inconsistent with mysql

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
SELECT UNIX_TIMESTAMP(1);
```

### 2. What did you expect to see? (Required)
result from mysql
```

MySQL [(none)]> SELECT UNIX_TIMESTAMP(1);
+-------------------+
| UNIX_TIMESTAMP(1) |
+-------------------+
| 0 |
+-------------------+
1 row in set, 1 warning (0.00 sec)

```

### 3. What did you see instead (Required)
result from tidb
```
MySQL [testdb_1746708390968]> SELECT UNIX_TIMESTAMP(date(1));
+-------------------------+
| UNIX_TIMESTAMP(date(1)) |
+-------------------------+
| NULL |
+-------------------------+
1 row in set, 1 warning (0.01 sec)

```

### 4. What is your TiDB version? (Required)
```
MySQL [testdb_1746708390968]> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v8.5.1
Edition: Community
Git Commit Hash: fea86c8e35ad4a86a5e1160701f99493c2ee547c
Git Branch: HEAD
UTC Build Time: 2025-01-16 07:38:34
GoVersion: go1.23.4
Race Enabled: false
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
```