[BUG][AVRO] object with only additionalProperties is represented with a map with items instead of values property

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

When generating avro schema associated to an openapi spec that contains an object with only additionalProperties set, the avro schema generated is not valid:

      "type": ["null", {
        "type": "map",
        "items": "string"
      }],

where it should be

      "type": ["null", {
        "type": "map",
        "values": "string"
      }],

openapi-generator version

openapi-generator-cli 7.10.0
  commit : 12dfe8f

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Sample API
  description: Optional multiline or single-line description in [CommonMark](http://commonmark.org/help/) or HTML.
  version: 0.1.9

servers:
  - url: http://api.example.com/v1
    description: Optional server description, e.g. Main (production) server
  - url: http://staging-api.example.com
    description: Optional server description, e.g. Internal staging server for testing

paths:
  /users:
    get:
      summary: Returns a list of users.
      description: Optional extended description in CommonMark or HTML.
      responses:
        "200": # status code
          description: A JSON array of user names
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string
components:
  schemas:
    Alert:
      type: object
      properties:
        tags:
          type: object
          description: Label-Value Pairs to identify properties about an alert
          additionalProperties:
            type: string
          example:
            cluster: prod
            type: unexpected_shutdown

Steps to reproduce

openapi-generator-cli generate -g avro-schema -i resources/test/openapi-spec.yaml -o test

Suggest a fix

I suppose it's related to

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/avro-schema/typeProperty.mustache

or at least that could be fixed by a new template but know about nothing about the way it works. I'd be happy to try to fix it if you could give me some pointers.

Thx!

Start with modules/openapi-generator/src/main/resources/avro-schema/typeProperty.mustache and reproduce the issue using resources/test/openapi-spec.yaml with the provided generate command. Compare the generated map schema with valid Avro syntax and verify that the output uses the expected map property.