disable strict sql mode should not ignore bad null error

[jackysp]

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.

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?

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

3. What did you see instead?
   No error returned.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   9d25a85 at master branch.

[shenli]

How do you find this bug?

[jackysp]

I'm porting MySQL tests.