NOT NULL should not change the query result

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

Case 1:
```SQL
CREATE TABLE t1 (c1 INT);
INSERT INTO t1 VALUES (1);
SELECT c1 FROM t1 WHERE (NULLIF(t1.c1, (t1.c1 IS NOT NULL) REGEXP('\\>{U'))); -- [HY000][1105] other error: Invalid regexp pattern
mysql> EXPLAIN SELECT c1 FROM t1 WHERE (NULLIF(t1.c1, (t1.c1 IS NOT NULL) REGEXP('\\>{U')));
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------------------------------+
| TableReader_7 | 8000.00 | root | | data:Selection_6 |
| └─Selection_6 | 8000.00 | cop[tikv] | | if(eq(test.t1.c1, regexp(cast(not(isnull(test.t1.c1)), var_string(20)), "\>{U")), NULL, test.t1.c1) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)
```
Case 2:
```SQL
CREATE TABLE t1 (c1 INT NOT NULL);
INSERT INTO t1 VALUES (1);
SELECT c1 FROM t1 WHERE (NULLIF(t1.c1, (t1.c1 IS NOT NULL) REGEXP('\\>{U'))); -- 1
mysql> EXPLAIN SELECT c1 FROM t1 WHERE (NULLIF(t1.c1, (t1.c1 IS NOT NULL) REGEXP('\\>{U')));
+-------------------------+----------+-----------+---------------+-----------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+-----------------------------------------+
| TableReader_7 | 8000.00 | root | | data:Selection_6 |
| └─Selection_6 | 8000.00 | cop[tikv] | | if(eq(test.t1.c1, 0), NULL, test.t1.c1) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+-----------------------------------------+
3 rows in set (0.00 sec)
```
### 2. What did you expect to see? (Required)
The same SELECT statement should both return value 1 or return an error, since the only difference between the two cases is whether the column c1 is NOT NULL or not. NOT NULL should not change the query result.
### 3. What did you see instead (Required)
Case 1 returns an error, case 2 return a value 1.
### 4. What is your TiDB version? (Required)

Release Version: v6.6.0
Edition: Community
Git Commit Hash: f4ca0821fb96a2bdd37d2fb97eb26c07fc58d4e4
Git Branch: heads/refs/tags/v6.6.0
UTC Build Time: 2023-02-17 14:49:02
GoVersion: go1.19.5
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: tikv