SHOW WARNINGS fails to fetch the warning message

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```sql
DROP TABLE IF EXISTS t0;
CREATE TABLE t0(c0 INT);
INSERT INTO t0 VALUES (1);
SELECT c0 FROM t0 WHERE EXP(244039520);
SHOW WARNINGS;
```

### 2. What did you expect to see? (Required)
SHOW WARNINGS should return the warning message, instead of an empty set.
### 3. What did you see instead (Required)
```sql
Empty set (0.00 sec)
```
### 4. What is your TiDB version? (Required)
```sql
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()

|
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.4.0
Edition: Community
Git Commit Hash: 55f3b24c1c9f506bd652ef1d162283541e428872
Git Branch: heads/refs/tags/v5.4.0
UTC Build Time: 2022-01-25 08:39:26
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```