Add No-Required params to custom REST API

I made a custom REST API on Magento 2. I want some params to be required and other to be no required or accept null. I have the next code:

Api/Order/ShippingInterface

 /** * API00004. Envío de Pedido * Url Path: /rest/V1/orders/sap/shipping * Se mandará a este servicio la información cuando un pedido ya ha * sido enviado. * * @api * @param string IdPedido * @param string ControlNumber * @param string UrlFile * @param string DecodeFile * @param string[] Trackings * @return string JSON con Mensaje de Confirmación o Error. */ public function set( $IdPedido = null, $ControlNumber = null, $UrlFile = null, $DecodeFile = null, $Trackings = null ); 

Model/Api/Order/Shipping.php

/** * API00004. Envío de Pedido * Url Path: /rest/V1/orders/sap/shipping * Se mandará a este servicio la información cuando un pedido ya ha * sido enviado. * * @api * @param string IdPedido * @param string ControlNumber * @param string UrlFile * @param string DecodeFile * @param string[] Trackings * @return string JSON con Mensaje de Confirmación o Error. */ public function set( $IdPedido = null, $ControlNumber = null, $UrlFile = null, $DecodeFile = null, $Trackings = null ){ ... } 

But when I send a Json like:

{"IdPedido":null,"ControlNumber":null,"UrlFile":null,"DecodeFile":null,"Trackings":null}

It returns me the next error:

message":"One or more input exceptions have occurred.","errors":[{"message":"%fieldName is a required field.","parameters":{"fieldName":"IdPedido"}},{"message":"%fieldName is a required field.","parameters":{"fieldName":"ControlNumber"}},{"message":"%fieldName is a required field.","parameters":{"fieldName":"UrlFile"}},{"message":"%fieldName is a required field.","parameters":{"fieldName":"DecodeFile"}},{"message":"%fieldName is a required field.","parameters":{"fieldName":"Trackings"}}]

How can I do this?

Thanks.