[BUG][KOTLIN] Model not generated when used as $ref in multipart/form-data requestBody

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 using openApiGenerate with a yaml, model is not generated when used as a $ref inside a multipart/form-data requestBody.

If multipart/form-data is changed to application/json, the model will be generated.

openapi-generator version

5.2.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: YAML for bug report
  version: '1.0'
  title: Bug report
tags:
  - name: Test
    description: Test
paths:

  /test/create:
    post:
      tags:
        - Test
      summary: Summary
      description: Description
      operationId: testCreate
      responses:
        '200':
          description: Successful operation
        '400':
          description: Invalid request
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              required:
                - file
                - json
              properties:
                file:
                  type: string
                  format: binary
                json:
                  $ref: '#/components/schemas/SomeRefObject'

components:
  schemas:
    SomeRefObject:
      description: 'Some Ref Object'
      type: object
      required:
        - id
      properties:
        id:
          type: string
          description: 'Some ID'

Generation Details

Gradle: 7.2
Kotlin: 1.5.30
Java: 11

Also tested with: SerializationLibrary: jackson (2.12.3)

Steps to reproduce

Create a gradle file with dependencies and the org.openapi.generator plugin

Execute: gradle openApiGenerate

Start with the supplied OpenAPI YAML and run the Gradle openApiGenerate command using the Kotlin generator. Compare generation for the multipart/form-data and application/json variants; done means SomeRefObject is generated for the multipart request body.