[BUG][Java] `$ref` with `multipart/form-data` is always null

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

As titled, when a property is referenced using $ref with multipart/form-data in the requestBody, such property is always null. It works otherwise when not using $ref, or when not using multipart/form-data. This issue might only occur with the above combinations.

openapi-generator version

I tested with docker images of openapi-generator-cli v5.2.0 and v5.4.0. Both have the same issue.
I couldn't test with the latest docker images v6.0.1. The server gives HTTP error 503 for all requests and refuses to work. This is also the reason why I couldn't test with the latest master. (This would be another issue.)

OpenAPI declaration file content or url

For the source codes to reproduce the issue, please refer to this repo:
https://github.com/jeffshee/openapi-test
The OpenAPI declaration file can be found here:
https://github.com/jeffshee/openapi-test/blob/master/api.yaml

Generation Details

Steps to reproduce

Docker environment is required for using the Makefile provided in the repo.

1. Clone the repo.
2. Execute make all to compile and run the docker image.
3. Test the APIs run at http://localhost:8080/v1. test1 is not working. For comparison, test2 and test3 are working.

Related issues/PRs

N/A

Suggest a fix

N/A

Start with the linked openapi-test repository and its api.yaml; run the Makefile's make all in Docker and compare test1 with test2 and test3 at localhost:8080/v1. Done when the generated Java behavior for the $ref multipart/form-data case matches the working non-reference cases.