Endpoints are not created for routes with Pydantic model default values

Start in openapi_python_client/parser/properties/union.py around line 87 and reproduce the failure with the supplied OpenAPI specification or FastAPI example. Trace how the union default is validated, then verify that the endpoint is generated successfully instead of being omitted.

Describe the bug
When attempting to generate the client for an endpoint containing a route with a default value represented by a Pydantic model, the client generation fails. Specifically, the issue arises when the default value is set as a Pydantic model, such as the example below:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()
class Cake(BaseModel):
    slices: int

@app.post("/eat_cake")
async def eat_cake(cake: Cake | None = Cake(slices=20)):
    return {"message": f"Eating {cake.slices} slices of cake!"}

The generated client results in the following error message:

Warning(s) encountered while generating. Client was generated, but some pieces may be missing
WARNING parsing POST /graph_study/{customer_name}/distinct_identities/distinct_to_base/eat_cake within default. Endpoint will not be generated.
Value {'slices': 20} is not valid, only None is allowed

This issue is caused by a condition not being met in the union.py file.

To Reproduce

1. Define a FastAPI app with a route containing a Pydantic model with a default value.
2. Attempt to generate the client using openapi-python-client version 0.17.2.

OpenAPI Spec File

{
  "openapi": "3.0.2",
  "info": {
    "title": "FastAPI",
    "version": "1.0"
  },
  "paths": {
    "/cake_eater/eat_cake": {
      "post": {
        "summary": "Eat Cake",
        "operationId": "eat_cake_cake_eater___eat_cake_post",
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "anyOf": [
                  {
                    "$ref": "#/components/schemas/Cake"
                  },
                  {
                    "type": "null"
                  }
                ],
                "title": "Cake",
                "default": {
                  "slices": 20
                }
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Successful Response",
            "content": {
              "application/json": {
                "schema": {}
              }
            }
          },
          "422": {
            "description": "Validation Error",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/HTTPValidationError"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Cake": {
        "properties": {
          "slices": {
            "type": "integer",
            "title": "Slices"
          }
        },
        "type": "object",
        "required": [
          "slices"
        ],
        "title": "Cake"
      },
      "HTTPValidationError": {
        "properties": {
          "detail": {
            "items": {
              "$ref": "#/components/schemas/ValidationError"
            },
            "type": "array",
            "title": "Detail"
          }
        },
        "type": "object",
        "title": "HTTPValidationError"
      },
      "ValidationError": {
        "properties": {
          "loc": {
            "items": {
              "anyOf": [
                {
                  "type": "string"
                },
                {
                  "type": "integer"
                }
              ]
            },
            "type": "array",
            "title": "Location"
          },
          "msg": {
            "type": "string",
            "title": "Message"
          },
          "type": {
            "type": "string",
            "title": "Error Type"
          }
        },
        "type": "object",
        "required": [
          "loc",
          "msg",
          "type"
        ],
        "title": "ValidationError"
      }
    }
  }
}

Desktop (please complete the following information):

• OS: Windows 11
• Python Version: 3.11.1
• openapi-python-client version 0.17.2

Additional context