`UNIX_TIMESTAMP` doesn't support all formats which are supported by 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 UNIX_TIMESTAMP('');
+--------------------+
| UNIX_TIMESTAMP('') |
+--------------------+
| 0.000000 |
+--------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('2024');
+------------------------+
| UNIX_TIMESTAMP('2024') |
+------------------------+
| 0.000000 |
+------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('20240228T123456');
+-----------------------------------+
| UNIX_TIMESTAMP('20240228T123456') |
+-----------------------------------+
| 1709094896 |
+-----------------------------------+
1 row in set (0.01 sec)

mysql> SELECT UNIX_TIMESTAMP('28 February 2024');
+------------------------------------+
| UNIX_TIMESTAMP('28 February 2024') |
+------------------------------------+
| 0.000000 |
+------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('2024-02-28abc');
+---------------------------------+
| UNIX_TIMESTAMP('2024-02-28abc') |
+---------------------------------+
| 1709049600 |
+---------------------------------+
1 row in set, 1 warning (0.00 sec)
```

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

```
mysql> SELECT UNIX_TIMESTAMP('');
+--------------------+
| UNIX_TIMESTAMP('') |
+--------------------+
| NULL |
+--------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('2024');
+------------------------+
| UNIX_TIMESTAMP('2024') |
+------------------------+
| NULL |
+------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('20240228T123456');
+-----------------------------------+
| UNIX_TIMESTAMP('20240228T123456') |
+-----------------------------------+
| NULL |
+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('28 February 2024');
+------------------------------------+
| UNIX_TIMESTAMP('28 February 2024') |
+------------------------------------+
| NULL |
+------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SELECT UNIX_TIMESTAMP('2024-02-28abc');
+---------------------------------+
| UNIX_TIMESTAMP('2024-02-28abc') |
+---------------------------------+
| NULL |
+---------------------------------+
1 row in set, 1 warning (0.00 sec)
```

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

Start by running the listed UNIX_TIMESTAMP queries against TiDB and MySQL, noting the differences for each input format. The issue names no implementation file or test; locate the existing UNIX_TIMESTAMP entry point and date-format tests, then add coverage so the listed cases match the expected MySQL results.

Written by the indexing model from the issue text.