[BUG] [DIO] Dart Diox Next - doesn't allow `null` as json response

Description

OpenApi schema allows for empty responses wich should not contain any data (see here). I would expect that in this case there is no actual consideration of the response body and we would at least allow an request with an optional Object like _dio.request<Object?>.
The API we are requesting from returns null in the response body, which then breaks the dart client client as it can not parse null to an object.

openapi-generator version

5.3.1

OpenAPI declaration file content or url

Actually any request which has a body containing null should create an exception. But here is our failing request.

  /core/customer/paymentcards:
    post:
      responses:
        "201":
          description: Created
          headers:
            Location:
              schema:
                type: string
      summary: Add new payment card
      operationId: Add new payment card
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required:
                - successUrl
                - failedUrl
                - clientName
              properties:
                successUrl:
                  type: string
                  description: >-
                    URL for redirect after successful registration of a new
                    card.
                    `cardId=` is added to URL.
                failedUrl:
                  type: string
                  description: URL for redirect when the registration fails.
                clientName:
                  type: string
                  description: Used by the server to identify the App in use
                clientOs:
                  type: string
                  description: iOS - 12.0 (string, required) - Stored serverside to make it
                    easier for the customer to distinguish his tokens in lists.
      security:
        - bearer: []

Generation Details

Dart Dio-Next.

Steps to reproduce

Set up an Api which responds with null as json in the response body. Create the client. Call the generated method and wait for the exception.

Suggest a fix

Use _dio.request<Object?> in appropriate cases

Start with the generated Dart Dio-Next method for the payment-card POST and trace its response parsing when the body is null. Reproduce the request described in the issue, then verify that nullable Object handling is used where appropriate; done means a null JSON response completes without a parsing exception.