[BUG][Java][resttemplate] object that consists only of additionalProperties leads to broken code generation since 7.12.0

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

After updating from 7.11.0 to 7.12.0 my code generation breaks. The given example (for me) has two issues:

1. The generated code does not compile anymore. The hashCode-method looks like this return Objects.hash(, additionalProperties);. It does compile as soon as you add at least one parameter to the SampleClass though. Then the hashCode method is correct.
2. The generated class does not extend HashMap<String, Object> anymore but has a parameter called "additionalProperties". I rely on the generated class being a map, which I thought is correct for a schema like that.

openapi-generator version

7.12

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: "TEST"
  version: 1.0.0
components:
  schemas:
    SampleObject:
      type: object
      additionalProperties:
        type: object

Generation Details

I'm using Java 21 and Gradle 8.13. Here is my config

{
  "dateLibrary": "java8",
  "useJakartaEe": true,
  "library": "resttemplate",
  "enumPropertyNaming": "legacy",
  "generateAliasAsModel": true
}

When registering the Gradle task I do set globalProperties.set(mapOf("models" to "")) so I only get my model file generated.

Steps to reproduce

• Create the example.yaml above
• Execute the code generation with the provided config
• Compare generated classes in 7.11 and 7.12
  • 7.12 does not compile and does not extend HashMap
  • 7.11 does compile and extends HashMap

Related issues/PRs

I assume that this has to do with this PR https://github.com/OpenAPITools/openapi-generator/pull/19706 taken from the 7.12's release notes.

Suggest a fix

I'm quite sure this is not supposed to happen like that, as the code does not even compile anymore. Or am I doing something wrong here?

Thank you very much!

Start with the example OpenAPI declaration and the resttemplate generation configuration, then compare the generated SampleObject from versions 7.11 and 7.12. Read related PR 19706 and trace the model-generation path it changed. Done means the generated class compiles, has a valid hashCode method, and extends HashMap<String, Object> for an additionalProperties-only schema.