[BUG][Java][Spring] allOf with nullable produces JsonNullable<Object>

Description

The generated code for definitions which include allOf annotation with nullable properties produces JsonNullable<Object> when the property type is defined in parent object.

public class UpdateFirmJson   {
  @JsonProperty("firmName")
  private JsonNullable<String> firmName = JsonNullable.undefined();

  @JsonProperty("addressId")
  private JsonNullable<Object> addressId = JsonNullable.undefined();
...

openapi-generator version

4.1.3

OpenAPI declaration file content or url

swagger: "2.0"

info:
  title: Test for JsonNullable
  version: 1.0.0

paths:
  /firm/{firmId}:
    patch:
      tags:
        - firm
      summary: Update a firm
      operationId: updateFirm
      consumes:
        - application/json
      parameters:
        - in: path
          name: firmId
          required: true
          description: "Firm ID"
          type: integer
          format: int64
        - in: body
          name: updateFirm
          description: Field to be updated
          schema:
            $ref: "#/definitions/UpdateFirm"
      responses:
        204:
          description: Updated
  
definitions:
  FirmProperties:
    properties:
      addressId:
        type: string

  UpdateFirm:
    allOf:
      - $ref: '#/definitions/FirmProperties'
      - properties:
          firmName:
            type: string
            x-nullable: true
          addressId:
            x-nullable: true

Command line used for generation

./gradlew openApiGenerate

Steps to reproduce

Just run the generator

Related issues/PRs

Nothing found.

Suggest a fix

Support deriving the type from the parent object, i.e. JsonNullabe<String>.

Start by running ./gradlew openApiGenerate with the supplied OpenAPI YAML and inspect the Java Spring generator path handling allOf and x-nullable properties. Trace how addressId inherits its type from FirmProperties, then verify the generated field uses JsonNullable rather than JsonNullable. Add coverage for this reproduction if the generator's existing tests provide a suitable location.