Unexpected result of `<=>` in prepared statement

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

Hi,

There are two equivalent queries in the following test case, which include a `<=>` operation. The right operator is NULL, so I expected the result to be false. However, the prepared statement returns true.

```
SET @b = -966582751;
PREPARE prepare_query FROM 'SELECT ((?)<=>(MAX(CAST(NULL AS TIME))))';
EXECUTE prepare_query USING @b; -- 1
DEALLOCATE PREPARE prepare_query;
SELECT ((-966582751)<=>(MAX(CAST(NULL AS TIME)))); -- 0
```

I tried in MySQL 9.4.0, both queries return 0.

### 2. What did you expect to see? (Required)

The prepared statement returns 0;

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

The prepared statement returns 1;

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

Release Version: v9.0.0-beta.2.pre-196-g312226f869\nEdition: Community\nGit Commit Hash: 312226f869d56922135e1c177cac6c90319c3134\nGit Branch: HEAD\nUTC Build Time: 2025-09-21 08:27:14\nGoVersion: go1.23.11\nRace Enabled: false\nCheck Table Before Drop: false\nStore: unistore\nKernel Type: Classic