Different beheviors when performing `add column` in adding expression default values feature

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
create table tt(a int);
alter table tt add column b datetime default (date_format(now(),'%Y-%m'));

create table t(a int);
insert into t value(1);
alter table t add column b datetime default (date_format(now(),'%Y-%m'));
```

### 2. What did you expect to see? (Required)
```
mysql> create table tt(a int);
Query OK, 0 rows affected (0.01 sec)
mysql> alter table tt add column b datetime default (date_format(now(),'%Y-%m'));
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> create table t(a int);
Query OK, 0 rows affected (0.02 sec)
mysql> insert into t value(1);
Query OK, 1 row affected (0.00 sec)
mysql> alter table t add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04' for column 'b' at row 1
```

### 3. What did you see instead (Required)
```

tidb> create table tt(a int);
Query OK, 0 rows affected (0.01 sec)
tidb> alter table tt add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04'

tidb> create table t(a int);
Query OK, 0 rows affected (0.01 sec)
tidb> insert into t value(1);
Query OK, 1 row affected (0.00 sec)
tidb> alter table t add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04'
```

### 4. What is your TiDB version? (Required)

master