[BUG] [JAVA] Referencing Object with additionalPorperties does not import HashMap in Model

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

Generating Model for a Object that references an object that has "additionalProperties" fails to generate the import for HashMap

openapi-generator version

affected Version(s): 4.0.0 to 4.0.3
Not affected: 3.3.4 (sets the field to null, so no import is needed here)

OpenAPI declaration file content or url

Snipped:

    WithAdditinalPropertiesDTO:
      type: object
      properties:
        id:
          type: string
      additionalProperties: {}

    BrokenDTO:
      type: object
      properties:
        id:
          type: string
        withAdditinalProperties:
          $ref: '#/components/schemas/WithAdditinalPropertiesDTO'

gist: https://gist.github.com/FloKaemmerer/54a9663183688b0d5658a8064c32f695

Command line used for generation

mvn clean compile

used pom: https://gist.github.com/FloKaemmerer/9789a0fc5bfca47b6c4cf50c7573434b

Steps to reproduce

Related issues/PRs

Suggest a fix

Start with the OpenAPI YAML snippet and the provided pom, then run mvn clean compile to reproduce the generated Java model problem. Inspect the generated BrokenDTO and its imports, using the gist examples as reference. Done when the generated model imports HashMap for the referenced additionalProperties object and compilation succeeds.