Incorrect result for CONV with NULL parameters

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
DROP TABLE t1;
CREATE TABLE IF NOT EXISTS t1 (c1 SMALLINT UNSIGNED);
INSERT INTO t1 VALUES (23);
SELECT CONV(c1, c1, TRUNCATE(c1, NULL)) FROM t1; -- actual:{23}, expected:{NULL}
```
### 2. What did you expect to see? (Required)
NULL value should be returned.
### 3. What did you see instead (Required)
23 was returned.
### 4. What is your TiDB version? (Required)

Release Version: v8.5.3
Edition: Community
Git Commit Hash: dc2548aac79a712265e831cff2a3a896bc0a5a38
Git Branch: HEAD
UTC Build Time: 2025-07-31 13:54:43
GoVersion: go1.23.8
Race Enabled: false
Check Table Before Drop: false
Store: tikv