message: core: don't print error without connection
  This sometimes produced an error on startup when it tried to send out a
  heartbeat even though a connection had not been added yet.
diff: @@ -259,6 +259,12 @@ bool MavsdkImpl::send_message(mavlink_message_t& message)
  std::lock_guard<std::mutex> lock(_connections_mutex);
  + if (_connections.empty()) {
  + // We obviously can't send any messages without a connection added, so
  + // we silently ignore this.
  + return true;
  + }
  +
  uint8_t successful_emissions = 0;
  for (auto& _connection : _connections) {
  const uint8_t target_system_id = get_target_system_id(message);
  