[BUG][C++][Pistache-Server] Incorrect fromJson generation with required items.

Created by: SalDiAngelus

Description

When a datatype references other datatypes as properties, the generated fromJson function does not deserialize required items.

openapi-generator version

4.0.0-beta3

OpenAPI declaration file content or url

openapi: 3.0.2
info:
    title: Bug Test
    version: 1.0.0
paths:
    /default:
        post:
            tags:
                - default
            requestBody:
                content:
                    application/json:
                        schema:
                            $ref: '#/components/schemas/DefaultRequest'
                required: true
            responses:
                '201':
                    description: Created
                    content:
                        application/json:
                            schema:
                                type: string
components:
    schemas:
        Item3:
            required:
                - s
            type: object
            properties:
                s:
                    type: string
        Item1:
            required:
                - x
                - 'y'
                - z
            type: object
            properties:
                x:
                    format: double
                    type: number
                'y':
                    format: double
                    type: number
                z:
                    format: double
                    type: number
        Item2:
            required:
                - i
                - j
                - k
            type: object
            properties:
                i:
                    format: double
                    type: number
                j:
                    format: double
                    type: number
                k:
                    format: double
                    type: number
        DefaultRequest:
            required:
                - item1
                - item2
                - item3
            type: object
            properties:
                item1:
                    $ref: '#/components/schemas/Item1'
                item2:
                    $ref: '#/components/schemas/Item2'
                item3:
                    $ref: '#/components/schemas/Item3'

Command line used for generation

java -jar .\openapi-generator-cli-4.0.0-beta3.jar generate -i default.yaml -g cpp-pistache-server -o pistache

Steps to reproduce

Run the above command with the included yaml. Notice that DefaultRequest::fromJson() doesn't deserialize anything. Then remove required items from DefaultRequest in the yaml and generate the server again. DefaultRequest::fromJson() will correctly check if the items exist and deserialize them.