insert_many method missing field values.
http://docs.peewee-orm.com/en/latest/peewee/api.html?highlight=insert_many#Model.insert_many Looks to be working fine:

```python
class Reg(db.Model):
    val = TextField(null=True)

Reg.create_table()
Reg.insert_many([{}, {'val': 'asdf'}], fields=[Reg.val]).execute()
print(list(Reg.select().tuples()))
# [(1, None), (2, 'asdf')]
```
peewee==3.16.0
Python 3.10.6

For the following code:
```
class TestModel(Model):
    class Meta:
        database = mysql_db
    test_field = CharField(null=True)

TestModel.create_table() 

insert_data = [
    {}, {
        'test_field': "hello"
    }
]

TestModel.insert_many(insert_data).execute()
```
Result:
```
id    test_field
1         NULL
2         NULL
```
The test_field field of the second record was not inserted.

The following is the modified code:
```
class TestModel(Model):
    class Meta:
        database = mysql_db
    test_field = CharField(null=True)

TestModel.create_table() 

insert_data = [{
    'test_field': None
},{
    'test_field': "hello"
}
]

TestModel.insert_many(insert_data).execute()
```
It worked
```
id    test_field
1         NULL
2         hello
```
Is it by design?
This goes against my instincts.
Thank you for working on such an easy-to-use ORM framework

