Port Null transform to use MessageId (#1497)

shotover/src/transforms/null.rs

@@ -46,8 +46,14 @@ impl Transform for NullSink {
 
  async fn transform<'a>(&'a mut self, mut requests_wrapper: Wrapper<'a>) -> Result<Messages> {
  for message in &mut requests_wrapper.requests {
+ let request_id = message.id();
+
+ // reuse the requests to hold the responses to avoid an allocation
  *message =
  message.to_error_response("Handled by shotover null transform".to_string())?;
+
+ // set the response to point to its corresponding request
+ message.set_request_id(request_id)
  }
  Ok(requests_wrapper.requests)
  }