message: return empty polygon if agent does not exist anymore
diff: @@ -75,6 +75,9 @@ inline void ComputeSafetyPolygon(
  LonDirectionMode directional = LonDirectionMode::AUTO) {
  // 1. Get the ego agent required values
  auto ego_agent = observed_world.GetEgoAgent();
  + if (!ego_agent)
  + return safe_poly;
  +
  auto ego_pose = ego_agent->GetCurrentPosition();
  auto ego_state = ego_agent->GetCurrentState();
  auto theta = ego_state(StateDefinition::THETA_POSITION);
  