[BUG] [JAVA] GSON client does not accept null in JSON for an optional array

[MarcMil]

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?
  An exception is thrown when the server does return a null for an optional array

Description

The Java GSON client does currently not accept a null for an optional array. Sadly, my server generates nulls instead of omitting the item. The client should accept it instead of throwing an exception

openapi-generator version

master

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Test API
  description: Test API
  version: '0.1'
paths:
  /api/jobs:
    get:
      summary: Gets a job
      operationId: getsJob
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Test'
components:
  schemas:
    Test:
      required:
        - id
      type: object
      properties:
        id:
          type: integer
          description: An id
          format: int64
          example: 1

Generation Details

Steps to reproduce

I'm generating the client with

<generatorName>java</generatorName>
<library>okhttp-gson</library>

Related issues/PRs

Suggest a fix