[Java] add missing nullable judgement when required property is true

[fanqiewanzi]

fix #18516

• Add judgment for nullable field to the required judgment.
• Add test for java okhttp-gson template to avoid regressions

Tests: openapi.yaml

openapi: 3.0.3
info:
  title: test
  description: Test API
  version: 1.0.1

paths:
  /test:
    get:
      responses:
        200:
          description: Valid response
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/SomeObject"

components:
  schemas:
    SomeObject:
      type: object
      required: 
        - ids
        - users
        - user
        - role
        - custom
      properties:
        ids:
          type: array
          nullable: true
          items:
            type: integer
        users:
          type: array
          nullable: true
          items:
            type: object
            properties:
              id:
                type: string
        user:
          type: object
          nullable: true
          properties:
            id:
              type: string
        role:
          type: string
          nullable: true
          enum:
            - admin
            - tenant
        custom:
          $ref: "#/components/schemas/customEnum"
    customEnum:
      type: string
      nullable: true
      enum:
        - custom    

run following command to generate: openapi-generator-cli generate -i openapi.yaml -o test -g java

And the code compiled successfully.

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