Nullable fields on Tortoise model are required in create route

[rgreen32]

Hello!, I have simple model with a nullable field:

from tortoise import fields, models

class Scan(models.Model):
    id = fields.IntField(pk=True)
    scan_data = fields.JSONField()
    time_stamp = fields.DatetimeField(auto_now=True, null=True)  
    class Meta:
        table = "scans"

router = TortoiseCRUDRouter(
    schema=models.Scan_Pydantic, 
    db_model=models.Scan
}

If I send some data to the post route, I expect that the "time_stamp" field will not need to be present in the payload because it is declared as nullable.

{
    "carrier": "FEDEX",
    "scan_data": {"scan_code":  "CD",  "scan_description":  "DELAY"}
}

but instead this fails with a 422 saying the "time_stamp" field is required.

I understand that adding the field with a null value work but I feel like declaring the field as nullable on model should make the field optional entirely in body of create requests.

I'd be happy to submit a pr for this change if there is support for it ! :)

[awtkns]

If you want to PR this you totally can. If not, I would be happy to pick up the implementation sometime in the future. This is somewhat related to #73