Problems with json_decode when sending the data to my php code I get null

I'm doing a registry of users in android (confirm that if you send the necessary values) but when you get to the php my variable $ user and $ pass become null even though in the mysql table if you insert the record but blank data.

Use Restlet client to emulate the operation of my php code (which I do not know why it does not work)

also sometimes sent data to my php code: link

the answer in both cases is this: successful connection (php connection file) Username and password equals null (input values).

<?php
  require_once("conexion.php");

  $data = empty($_POST) ? json_decode(file_get_contents('php://input'), 
  true) : $_POST;

  $usuario = $data["usuario"];

  $pass = $data["contrasena"];


  $respuesta = array(
  "resultado"=> false,
  "mensaje" => ""
  );

  if($usuario==null&&$pass==null){
      echo "Usuario y contraseña es igual  a null";
  }
  else{
      $sql_insert="insert into usuarios VALUES (null,'$usuario','$pass')";


      $result=mysqli_query($conexion,$sql_insert);


          if($result){
              $respuesta["resultado"]=true;
              $respuesta["mensaje"]="Se guardo la información";
          }else{
              $respuesta["mensaje"]="Error";
          }


       echo json_encode($respuesta);
   }
?>

add an if to validate that $ user and $ pass are not empty since it inserts but carries them with null values.

I hope you can help me, thank you.