default value of column with no default value behavior is not compatible with mysql 8.0.29

[crazycs520]

Bug Report

I found this issue when investigating #50043

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (id int key, a int, b enum('a', 'b'));
alter table t alter column a drop default;
alter table t alter column b drop default;
insert ignore into t set id = 4;
show warnings;
insert ignore into t set id = 6, a = DEFAULT, b = DEFAULT;
show warnings;

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

From MySQL 8.0.29

the show warnings result should be:

> show warnings;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1364 | Field 'a' doesn't have a default value |
+---------+------+----------------------------------------+

And the table data should be:

> select * from t;
+----+--------+--------+
| id | a      | b      |
+----+--------+--------+
| 4  | <null> | <null> |
| 6  | <null> | <null> |
+----+--------+--------+

3. What did you see instead (Required)

the show warnings result is:

> show warnings;
+---------+------+---------------------------+
| Level   | Code | Message                   |
+---------+------+---------------------------+
| Warning | 1048 | Column 'a' cannot be null |
+---------+------+---------------------------+

And the table data is:

> select * from t;
+----+---+--------+
| id | a | b      |
+----+---+--------+
| 4  | 0 | <null> |
| 6  | 0 | <null> |
+----+---+--------+

4. What is your TiDB version? (Required)

+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v8.0.0-alpha-301-g1438892b9f-dirty       |
| Edition: Community                                        |
| Git Commit Hash: 1438892b9fed5efc6fae8df8d44c239368ced262 |
| Git Branch: master                                        |
| UTC Build Time: 2024-02-26 09:58:28                       |
| GoVersion: go1.21.5                                       |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: unistore                                           |
+-----------------------------------------------------------+