[BUG] Nullable and blank allowed properties result in incorrectly generated enum check

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using a generated DRF Spectacular schema which includes properties that are both nullable and can be blank, the Python generated client has an incorrect <enum-name>_validate_enum method in the object model class.

The method is:

@field_validator('my_type')
def my_type_validate_enum(cls, value):
    """Validates the enum"""
    if value is None:
        return value

    if value not in (0, 1, 2, 3, 4, 5, 6, null):
        raise ValueError("must be one of enum values (0, 1, 2, 3, 4, 5, 6, null)")
    return value

However, the null value is not known. I think this should be parsed to None by the generator.

openapi-generator version

7.1.0 - Python

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: My Example API
  version: 1.0.0
paths:
  /objects/:
    post:
      operationId: objects_create
      tags:
      - objects
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/MyObject'
      responses:
        '201':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/MyObject'
          description: ''
components:
  schemas:
    MyObject:
      type: object
      properties:
        myType:
          enum:
          - 0
          - 1
          - 2
          - 3
          - 4
          - 5
          - 6
          - null
          type: integer
          nullable: true
          minimum: 0
          maximum: 2147483647

Generation Details

Nothing special, I just use the docker image with options: `generate -i schema.yaml -g python -o openapi-generator/python/ -c config.json

Steps to reproduce

Generate any schema with a - null option in the enum list, this will be added as is to the if statement in the Python client.

Related issues/PRs

I could not find any.

Suggest a fix

The null option should be converted to the known Python None value. My guess would be that the issue is here: https://github.com/OpenAPITools/openapi-generator/blob/dd4143b75e58aaea4ba8a3d6b598386b550ffda8/modules/openapi-generator/src/main/resources/python/model_generic.mustache#L55-L81

Start with modules/openapi-generator/src/main/resources/python/model_generic.mustache around lines 55-81, then generate a Python client from the YAML schema in the issue using the Python generator. Check the generated enum validator for the nullable enum value. Done means the generated validation code uses Python's known None value rather than null and still handles the listed numeric values.