[BUG] typescript-fetch since 7.4.0 ignores nullable: true

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

Hello since update to 7.4.0, nullable: true anotation is ignored and typescript generated interface doesn´t include "| null"

openapi-generator version

7.4.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: TEST
  description: TEST
  version: 1.0.0
servers:
  - url: /
    description: Default Server URL
paths:
  /api/test:
    post:
      tags:
        - Test
      operationId: test
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/ResponseObject'
      responses:
        '200':
          description: OK
          content:
            '*/*':
              schema:
                $ref: '#/components/schemas/ResponseObject'

components:
  schemas:
    ResponseObject:
      type: object
      properties:
        code:
          type: integer
          nullable: true

Generation Details

I generate it using npm package: "@openapitools/openapi-generator-cli": "^2.13.1".
With command npx openapi-generator-cli generate with following configuration

{
    "$schema": "./node_modules/@openapitools/openapi-generator-cli/config.schema.json",
    "spaces": 4,
    "generator-cli": {
        "version": "7.4.0",
        "useDocker": true,
        "generators": {
            "v3.0": {
                "generatorName": "typescript-fetch",
                "glob": "openapi/*.{json,yaml}",
                "output": "src/openapi/#{name}",
                "typeMappings": {
                    "Date": "string"
                },
                "additionalProperties": {
                    "prefixParameterInterfaces": true,
                    "useSingleRequestParameter": false,
                    "supportsES6": true
                }
            }
        }
    }
}

Steps to reproduce

If you generate source code from this provided yaml with 7.4.0 you will get generated output where ResponseObject.ts is defined like this:

export interface ResponseObject {
    /**
     * 
     * @type {number}
     * @memberof ResponseObject
     */
    code?: number;
}

but with all previouse versions it was like this

export interface ResponseObject {
    /**
     * 
     * @type {number}
     * @memberof ResponseObject
     */
    code?: number | null;
}

It looks like version 7.4.0 started to ignore nullable: true anotation.

Related issues/PRs

Maybe related to this issue ? #18005

Suggest a fix

Output should look like this:

export interface ResponseObject {
    /**
     * 
     * @type {number}
     * @memberof ResponseObject
     */
    code?: number | null;
}

Start by running the supplied OpenAPI YAML through the typescript-fetch generator at version 7.4.0 and compare the generated ResponseObject.ts with output from a previous version. Trace how nullable: true is handled during generation; done means the generated code includes number | null and the regression is covered by an appropriate generator test.