[BUG] [Python] nullable reference not generating any_of_x

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

I'm still having problems with the nullable reference. I've used the anyOf trick which passes the specification validation, but fails at runtime because the models.any_of_tier module is not generated.

ModuleNotFoundError: No module named '<my_package>.openapi_server.models.any_of_tier'

openapi-generator version

I just pulled the latest docker image.

OpenAPI declaration file content or url

        tier:
          anyOf:
            - $ref: '#/components/schemas/Tier'
          nullable: True

(for YAML code) or

Command line used for generation

docker run --rm -v ${BUILD_DIR}:/local openapitools/openapi-generator-cli generate
-i local/${ENDPOINT_SPEC}
-g python-flask
-o /local

Steps to reproduce

submit a request with tier: null

Related issues/PRs

https://github.com/OAI/OpenAPI-Specification/issues/1368

Suggest a fix

I've also tried:
alllOf and get
validation error: None is not of type 'object'
which makes sense because tier doesn't exist.

oneOf generates the same error as anyOf.
models.one_of_tier
ModuleNotFoundError: No module named '<my_package>.openapi_server.models.one_of_tier'

Start by running the issue's Docker command with the shown OpenAPI declaration and the python-flask generator, then inspect the generated models and the runtime error for tier: null. The fix is done when a nullable anyOf reference generates the needed model handling and a request containing tier: null no longer raises ModuleNotFoundError.