[PHP] Codegen generates models for schemas that are not objects

Description

I have an endpoint that returns a User object:

paths:
  /users/{id}:
      # ...
      responses:
        200:
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'

And models:

components:
  schemas:
    User:
      type: object
      properties:
        id: 
          $ref: '#/components/schemas/UserId'
        # other properties
    UserId:
      type: string
      format: uuid

This tool generates an empty model class for UserId, i.e. a class without properties. This makes things unusable because when deserializing API response to User object it sets id property to empty UserId object.
IMO we should not generate classes for models that are not objects. If a property is a reference to the model that is not an object then we should inherit a type from this model.

Example from PHP:
Current behaviour

class User implements ModelInterface, ArrayAccess
{
    protected static $swaggerTypes = [
        'id' => '\Example\Model\UserId'
    ];

    // ...
}

class UserId implements ModelInterface, ArrayAccess
{
    protected static $attributeMap = [
    ];

    public function __construct(array $data = null)
    {
    }

    // ...
}

Expected behaviour

class User implements ModelInterface, ArrayAccess
{
    protected static $swaggerTypes = [
        'id' => 'string'
    ];

    // ...
}

Swagger-codegen version

swaggerapi/swagger-codegen-cli-v3:3.0.61

Swagger declaration file content or url

Command line used for generation

docker run --rm -v "${PWD}" swaggerapi/swagger-codegen-cli-v3:3.0.61 generate -i /swagger.yml -l php

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Start with the supplied OpenAPI schema and the PHP generation command using swagger-codegen-cli-v3:3.0.61. Trace how referenced non-object schemas are handled during PHP model generation; done means UserId is not emitted as an empty model and User.id uses the underlying string type.