The result of using the plan of TableDual with the statement of NULL-safe equal is wrong

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t2(a int,b year,key(b));
insert into t values(null,null);
select * from t where b <=> null;

The plan is

+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| TableDual_5 | 0.00    | root |               | rows:0        |
+-------------+---------+------+---------------+---------------+

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

+------+------+
| a    | b    |
+------+------+
| NULL | NULL |
+------+------+

3. What did you see instead (Required)

Empty set

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-2807-g289dcfefd-dirty
Edition: Community
Git Commit Hash: 289dcfe
Git Branch: master
UTC Build Time: 2021-05-06 15:11:16
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[sylzd]

/assign

[ti-srebot]

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