0000-00-00 for date type cannot be select with is null

[lysu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql> create table t1 (f1 date not null);
Query OK, 0 rows affected (0.02 sec)

mysql> set sql_mode='';
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values('0000-00-00');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t1 where f1 is null;

2. What did you expect to see?

in mysql 5.7 will got

mysql> select * from t1 where f1 is null;
+------------+
| f1         |
+------------+
| 0000-00-00 |
+------------+
1 row in set (0.00 sec)

3. What did you see instead?

in TiDB

mysql> select * from t1 where f1 is null;
Empty set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v3.0.0-rc.1-85-g320d948e7
Git Commit Hash: 320d948e7d5c4bb03f947ff68a874985f36360da
Git Branch: test-master-tpcc
UTC Build Time: 2019-05-20 07:14:56
GoVersion: go version go1.12.1 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[lysu]

this question is found in test https://bugs.mysql.com/bug.php?id=14186