message: dont emit datagram event if bytes written returns 0
diff: @@ -369,11 +369,13 @@ impl<'a, 'sub, Config: endpoint::Config> tx::Message for ConnectionTransmission<
  let datagram_len = initial_capacity - encoder.capacity();
  self.context.path_mut().on_bytes_transmitted(datagram_len);
  + if datagram_len > 0 {
  self.context
  .publisher
  .on_datagram_sent(event::builders::DatagramSent {
  len: datagram_len as u16,
  });
  + }
  datagram_len
  }
  