[BUG] Conversion from v2 to v3 incorrectly changes additionalProperties: {} into additionalProperties: type: object

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

I have an OpenAPI v2 yaml that is being converted to v3 by openapi-generator-cli, and it ends up changing the behavior of my models from allowing any freeform additional properties of any type, to only allowing properties of type object.
Basically, it should be treating additionalProperties: {} just like it treats additionalProperties: true, but instead it converts additionalProperties: {} incorrectly.

openapi-generator version

v7.8.0

OpenAPI declaration file content or url

My openapi v2 yaml includes:

  protobufAny:
    type: object
    properties:
      '@type':
        type: string
    additionalProperties: {}

Generation Details

openapi-generator generate -i ./awesome.swagger.yaml -g openapi-yaml -o ./openapiv3

This is converted by openapi-generator into this yaml:

components:
  schemas:
    protobufAny:
      additionalProperties:
        type: object
      properties:
        '@type':
          type: string
      type: object

Instead it should be generating this following:

components:
  schemas:
    protobufAny:
      additionalProperties: true
      properties:
        '@type':
          type: string
      type: object

Steps to reproduce

openapi-generator generate -i ./awesome.swagger.yaml -g openapi-yaml -o ./openapiv3
Using this openapi v2 yaml

Suggest a fix

This should be generated instead:

components:
  schemas:
    protobufAny:
      additionalProperties: true
      properties:
        '@type':
          type: string
      type: object

Start by reproducing the issue with the provided openapi-generator generate command and the linked OpenAPI v2 YAML. Compare how additionalProperties: {} and additionalProperties: true are converted in the generated OpenAPI v3 YAML. Done means the empty object is emitted as additionalProperties: true without changing other schema fields.