prepared-plan-cache: return wrong results using "<=>" statement

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

prepared-plan-cache.enabled=true

use test;
drop table if exists t;
create table t(col1 enum("aa", "bb"), col2 int, index(col1, col2));
insert into t values("aa", 333);
prepare stmt from 'SELECT * FROM t t1 JOIN t t2 ON t1.col1 = t2.col1 WHERE t1.col1 <=> NULL';
execute stmt;
execute stmt;

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

mysql> execute stmt;
Empty set (0.01 sec)

3. What did you see instead (Required)

mysql> execute stmt;
+------+------+------+------+
| col1 | col2 | col1 | col2 |
+------+------+------+------+
| aa   |  333 | aa   |  333 |
+------+------+------+------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

Release Version: v5.4.0-alpha-217-gf45a8f051
Edition: Community
Git Commit Hash: f45a8f0511156855d2c109d216acdd1f900abe20
Git Branch: master
UTC Build Time: 2021-11-24 06:52:38
GoVersion: go1.16.5
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[Reminiscent]

#30003 can solve this problem. And I will add more test cases.

[github-actions]

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