[BUG] typescript-angular api.service does not respect nullable: true returnType

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

Adding nullable: true to a return type of a service is not respected in the generated client code.

openapi-generator version

in package.json
"@openapitools/openapi-generator-cli": "^2.7.0",
and in openapitools.json
"generator-cli": {
    "version": "7.0.1"
  }

OpenAPI declaration file content or url

openapi: 3.0.0
paths:
  /api/employer/getForUser:
    get:
      operationId: EmployerController_getForUser
      parameters: []
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                allOf:
                  - $ref: '#/components/schemas/ApiResponseEmployerAppModel'
                nullable: true

      tags:
        - employer
      security:
        - bearer: []
 
info:
  title: API
  description: API Documentation
  version: 1.0.0
  contact: {}
tags: []
servers: []
components:
  securitySchemes:
    bearer:
      scheme: bearer
      bearerFormat: JWT
      type: http
  schemas:
    ApiResponseEmployerAppModel:
      type: object
      properties:
        name:
          type: string

Generation Details

Steps to reproduce

using editor.swagger.io

1. use editor.swagger.io to generate typescript-angular client code for the provided spec
2. return type of service method is not nullable

using the openapi-generator tool

openapi-generator-cli generate -i PATH_TO_SPEC -g typescript-angular -o ./src/app/api --additional-properties=ngVersion=15.2.1,supportsES6=true,withInterfaces=true,stringEnums=true --skip-validate-spec --reserved-words-mappings long=lon

Related issues/PRs

Suggest a fix

I suspect that a check for the isNullable property can be added in the mustache template:
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-angular/api.service.mustache#L176C687-L176C687

Start with modules/openapi-generator/src/main/resources/typescript-angular/api.service.mustache, especially the referenced line around 176, and reproduce the issue with the provided OpenAPI schema and typescript-angular generation command. Compare the generated service return type with the nullable response declaration; done means the generated client preserves nullable: true in that return type.