"TypeError: Cannot instantiate typing.Union" with "OneToOneNullableRelation"

Describe the bug
We get a TypeError when the field OneToOneNullableRelation is used.

Traceback (most recent call last):
  File "/root/senran_dev/run.py", line 5, in <module>
    from bot import SenranBot
  File "/root/senran_dev/bot.py", line 10, in <module>
    import db
  File "/root/senran_dev/db/__init__.py", line 1, in <module>
    from .models import *
  File "/root/senran_dev/db/models.py", line 21, in <module>
    class User(BaseModel):
  File "/root/senran_dev/db/models.py", line 23, in User
    activated_guild = fields.OneToOneNullableRelation(model_name='models.Guild')
  File "/usr/lib/python3.8/typing.py", line 729, in __call__
    result = self.__origin__(*args, **kwargs)
  File "/usr/lib/python3.8/typing.py", line 339, in __call__
    raise TypeError(f"Cannot instantiate {self!r}")
TypeError: Cannot instantiate typing.Union

Process finished with exit code 1

To Reproduce
Create a model with a OneToOneNullableRelation relation

Expected behavior
The model and the field should work properly.

Additional context
Nothing else to add.

Start with the OneToOneNullableRelation declaration shown in db/models.py and trace the corresponding fields implementation in Tortoise ORM. Reproduce the Python 3.8 TypeError with a model containing this relation; done means the model imports successfully and the field works without attempting to instantiate typing.Union.