text: Title: unique indexes (for unique_together) on nullable columns need to be optionable for nulls not distinct
  Body: assume that i have two models `shop`, `customer`. each shop can have only one customer with an unique phone number till deletes it. please pay attention that customer's phone_number is not unique in column. that is unique together with its shop and delete state.
  
  ```py
  from tortoise import Model, fields
  
  class Base(Model):
    deleted_at = fields.DateTimeField(null=True)
    class Meta:
      abstract = True
  
  class Shop(Base):
    title = fields.CharField(...)
  
  class Customer(Base):
    phone_number = fields.CharField(...) # not used unique here as you expect
    shop = fields.ForeignKeyField('models.Shop', 'customers', fields.CASCADE)
    class Meta:
      unique_together = ('shop_id', 'phone_number', 'deleted_at')
  ```
  but this simple code is not work as i expected
  in postgres that i tested, created index did'nt follow by `nulls not distinct`
  when not follow. unique index considers nulls not equal.
  tortoise version : 0.21.3