set variable is inconsistent with mysql

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

set @b = aa;

### 2. What did you expect to see? (Required)
Consistent with mysql 8.0:

set @b = aa;
ERROR 1054 (42S22): Unknown column 'aa' in 'field list'

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

set @b = aa;
Query OK, 0 rows affected (0.00 sec)
### 4. What is your TiDB version? (Required)

Release Version: v7.4.0-alpha-21-gcd4cbcd742
Edition: Community
Git Commit Hash: cd4cbcd7422f133d88ffc87a41d077d9d8b07bb1
Git Branch: master
UTC Build Time: 2023-08-07 08:11:41
GoVersion: go1.20.5
Race Enabled: false
Check Table Before Drop: false
Store: unistore |