Violation of the not-null constraint for adding column

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (a tinyint);
insert into t set a = 10 ;
alter table t add column b int not null, change column a c char(5) first;
select * from t;

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

+------+---+
| c    | b |
+------+---+
| 10   | 0 |
+------+---+

3. What did you see instead (Required)

+------+---+
| c    | b |
+------+---+
| 10   | NULL |
+------+---+

4. What is your TiDB version? (Required)

405d2c0