disable strict sql mode should not ignore bad null error

[jebter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (i int not null);
set sql_mode = '';
insert into t values (null);

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

ERROR 1048 (23000) at line 4: Column 'i' cannot be null

3. What did you see instead (Required)

No error returned.

4. What is your TiDB version? (Required)

Release Version: v5.0.0-nightly\nEdition: Community\nGit Commit Hash: e2740f5\nGit Branch: heads/refs/tags/v5.0.0-nightly\nUTC Build Time: 2021-04-02 16:25:23\nGoVersion: go1.13\nRace Enabled: false\nTiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306\nCheck Table Before Drop: false

[jebter]

recurrent bug：#10153

[wjhuang2016]

It's the expected behavior, refer to https://bugs.mysql.com/bug.php?id=101727