[BUG] [typescript] Model for oneOf types not being generated.

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?

Description

The generated typescript client API does not include files for types with oneOf schemas. The API class includes the import, but the model is missing.

openapi-generator version

5.0.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: 1.0.0
  title: "Typescript client oneOf Test"
tags:
  - name: Test
    description: "Test API"
paths:
  "/test":
    get:
      tags:
        - Test
      operationId: test
      responses:
        "200":
          description: "Test response"
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Test"
components:
  schemas:
    A:
      type: object
      properties:
        hello: 
          type: string
    B:
      type: object
      properties:
        world: 
          type: string
    Test:
      type: array
      items:
        oneOf:
          - $ref: "#/components/schemas/A"
          - $ref: "#/components/schemas/B"

Generation Details & Steps to reproduce

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g typescript -i ./api.yml -o ./src --additional-properties=supportsES6=true,useObjectParameters=true

Start with the provided OpenAPI YAML and run the listed openapi-generator CLI command for the typescript generator. Compare the API import with the generated model files; the work is done when schemas using oneOf produce the expected model file and the generated client remains usable.