unique indexes (for unique_together) on nullable columns need to be optionable for nulls not distinct

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.

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

No source file or test is named. Start by tracing how Meta.unique_together is converted into PostgreSQL unique indexes, then inspect the generated index for nullable columns. Done means an option can request NULLS NOT DISTINCT for this constraint and the behavior is covered for the shop, customer, phone_number, and deleted_at example.