[BUG][Java/spring] Incorrect Handling of Nullable Fields in OpenAPI Generator

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?
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Description

Essentially, I have one endpoint to fetch user data (see below). What I'm attempting to achieve is to make phoneNumber optional, or set it as nullable: true. Therefore, I have two options:

1. Omit mentioning that field in the required section.
2. Use nullable: true at the field level.

In addition, I have a method that converts a UserDTO (database object) to a RestUser (generated model). Take a look at this method:

public static RestUser toRestUser(UserDTO userDTO) {

    return new RestUser()
        .id(userDTO.getId())
        .name(userDTO.getName())
        .phoneNumber(userDTO.getPhoneNumber())
        .email(userDTO.getEmailAddress());
}

1. If I don't specify that field as required in Swagger, the generator will create that field with Optional.of. At that point, if my value is null, it will throw a NullPointerException. In my opinion, we should use Optional.ofNullable for non-required fields.
2. If I use nullable: true on the field, I get a completely different value. If I pass a value to a nullable field, I receive this below result.

Actual output

However, this is incorrect why getting "present": true; the corrected value should be null if i didn't pass the value.

{
  "id": 1,
  "name": "John",
  "phoneNumber": {
    "present": true
  },
  "mail": "test@gmail.com"
}

Expected output

{
  "id": 1,
  "name": "John",
  "phoneNumber": null,
  "mail": "test@gmail.com"
}

openapi-generator version

7.2.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Backend
  version: 1.0.0
  
paths:
  /users:
    get:
      tags:
       - user
      summary: get.users
      description: get all users.
      responses:
        200:
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
          
components:
  schemas:
    User:
      type: object
      properties:
        id: 
          type: integer
        name:
          type: string
        phoneNumber:
          type: integer
          format: int64
          nullable: true
        email:
          type: string

Start with the supplied OpenAPI declaration and reproduce generation for the User model using version 7.2.0, then inspect the generated Java RestUser/User model behavior for omitted and nullable phoneNumber values. Compare the serialized output with the expected null value and verify both non-required and nullable cases after the behavior is corrected.