[BUG] [JAVA] Validate Required fields nullable

Bug Report Checklist

• [ X] Have you provided a full/minimal spec to reproduce the issue
• [X ] Have you validated the input using an OpenAPI validator (https://editor.swagger.io/)
• [ X] Have you tested with the latest master to confirm the issue still exists
• [X ] Have you searched for related issues/PRs
• [ X] What's the actual output vs expected output
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

In validateJsonElement method, during verify openapiRequiredFields jsonElement maybe JsonNull.
jsonElement.getAsJsonObject() is right for JsonObject not JsonElement.

if (jsonElement.getAsJsonObject().get(requiredField) == null) {
throw new IllegalArgumentException(String.format("The required field %s is not found in the JSON string: %s", requiredField, jsonElement.toString()));
}

openapi-generator version

7.10.0

OpenAPI declaration file content or url

openapi: 3.0.0
servers:

• url: 'http://petstore.swagger.io/v2'
  info:
  version: 1.0.0
  title: OpenAPI Petstore
  tags:
• name: pet
  description: Everything about your Pets
  paths:
  /pet:
  post:
  tags:
  - pet
  summary: Add a new pet to the store
  description: ''
  operationId: addPet
  responses:
  '200':
  description: successful operation
  content:
  application/xml:
  schema:
  $ref: '#/components/schemas/Pet'
  application/json:
  schema:
  $ref: '#/components/schemas/Pet'
  requestBody:
  $ref: '#/components/requestBodies/Pet'
  components:
  requestBodies:
  Pet:
  content:
  application/json:
  schema:
  $ref: '#/components/schemas/Pet'
  application/xml:
  schema:
  $ref: '#/components/schemas/Pet'
  description: Pet object that needs to be added to the store
  required: true
  schemas:
  Pet:
  title: a Pet
  description: A pet for sale in the pet store
  type: object
  required:
  - name
  - settings
  properties:
  id:
  type: integer
  format: int64
  name:
  type: string
  example: doggie
  settings:
  $ref: '#/components/schemas/settings'
  xml:
  name: Pet
  settings:
  title : settings
  nullable : true
  properties:
  id:
  type: integer
  format: int64
  name:
  type: string
  example: doggie

Generation Details

-g java
--additional-properties disallowAdditionalPropertiesIfNotPresent=false
-o build3
--strict-spec false
--skip-validate-spec

Steps to reproduce

Related issues/PRs

Suggest a fix

check jsonElement is JsonObject

if (jsonElement.getAsJsonObject().get(requiredField) == null) {
throw new IllegalArgumentException(String.format("The required field %s is not found in the JSON string: %s", requiredField, jsonElement.toString()));
}

Start by locating validateJsonElement in the Java generator output or its template, then reproduce the supplied OpenAPI Pet/settings case with the listed -g java options. Verify that a nullable required settings value is handled without treating JsonNull as a JsonObject, and confirm the generated client reports missing fields correctly.