[BUG] [Typescript Angular] generates invalid project when there are no models

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 an api that has no models the typescript-angular generator will generate an empty models.ts file which is then imported in index.ts. This causes a compile error when building the client

'model/models.ts' is not a module.

openapi-generator version

6.2.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: 'Demo'
  version: '1.0'
paths:
  /doit:
    get:
      tags:
        - Tag
      operationId: doSomething
      responses:
        '204':
          description: No Content
components: { }

Generation Details

{
  "$schema": "node_modules/@openapitools/openapi-generator-cli/config.schema.json",
  "spaces": 2,
  "generator-cli": {
    "version": "6.2.0",
    "generators": {
      "ng-client": {
        "generatorName": "typescript-angular",
        "glob": "swagger.yaml",
        "output": "#{cwd}/dist",
        "additionalProperties": {
          "npmName": "test-client",
          "npmVersion": "0.0.1",
          "npmRepository": "https://npm.pkg.github.com",
          "providedIn": "root"
        }
      }
    }
  }
}

Steps to reproduce

generate the typescript project and then attempt to compile it

Related issues/PRs

Suggest a fix

model.ts should only be generated and imported if there are models

Generate the supplied OpenAPI declaration with the typescript-angular generator, then compile the generated client to reproduce the invalid models.ts import from index.ts. Inspect how model files and the index export are generated for specs with no components.models. Done means the no-model client compiles without an empty models.ts module or an invalid import.