Invalid code generated for nullable discriminated union

[codebutler]

Describe the bug

Given the schema below, using the generated code like:

from test_client.models.demo import Demo
from test_client.models.a import A

Demo(example_union=A()).to_dict()

fails with:

Traceback (most recent call last):
  File "/Users/eric/Desktop/test/test.py", line 4, in <module>
    Demo(example_union=A()).to_dict()
  File "/Users/eric/Desktop/test/test_client/models/demo.py", line 32, in to_dict
    elif isinstance(self.example_union, Union["A", "B"]):
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eric/.pyenv/versions/3.12.1/lib/python3.12/typing.py", line 1564, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eric/.pyenv/versions/3.12.1/lib/python3.12/typing.py", line 1568, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union

OpenAPI Spec File

openapi: 3.0.3
info:
  title: Test
  version: 0.0.0
  description: test
paths: {}
components:
  schemas:
    Demo:
      type: object
      properties:
        example_union:
          allOf:
            - $ref: '#/components/schemas/ExampleUnion'
          nullable: true # <-- bug does not happen if this line is removed
    A:
      type: object
      properties:
        type:
          type: string
    B:
      type: object
      properties:
        type:
          type: string
    ExampleUnion:
      oneOf:
      - $ref: '#/components/schemas/A'
      - $ref: '#/components/schemas/B'
      discriminator:
        propertyName: type
        mapping:
          a: '#/components/schemas/A'
          b: '#/components/schemas/B'

Desktop

• OS: macOS 14.3
• Python Version: 3.12.1
• openapi-python-client version: 0.17.2

Additional context

The failing generated code is:

isinstance(self.example_union, Union["A", "B"])

Using instanceof on a Union with quoted types is not allowed:

>>> class A:
...     pass
...
>>> class B:
...     pass
...
>>> from typing import Union
>>> isinstance(None, Union[A, B])
False
>>> isinstance(None, Union["A", "B"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/eric/.pyenv/versions/3.12.1/lib/python3.12/typing.py", line 1564, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eric/.pyenv/versions/3.12.1/lib/python3.12/typing.py", line 1568, in __subclasscheck__
    if issubclass(cls, arg):
       ^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union