Use id="empty" to differentiate from a not set octomap field

moveit_core/planning_scene/src/planning_scene.cpp

@@ -842,6 +842,12 @@ bool PlanningScene::getOctomapMsg(octomap_msgs::OctomapWithPose& octomap) const
     ROS_ERROR_NAMED(LOGNAME, "Unexpected number of shapes in octomap collision object. Not including '%s' object",
                     OCTOMAP_NS.c_str());
   }
+  else
+  {
+    // indicate empty octomap, which is different from a not set octomap field
+    octomap.octomap.id = "empty";
+    return true;
+  }
   return false;
 }
 
@@ -1289,7 +1295,7 @@ bool PlanningScene::setPlanningSceneDiffMsg(const moveit_msgs::PlanningScene& sc
     result &= processCollisionObjectMsg(collision_object);
 
   // if an octomap was specified, replace the one we have with that one
-  if (!scene_msg.world.octomap.octomap.data.empty())
+  if (!scene_msg.world.octomap.octomap.id.empty())
     processOctomapMsg(scene_msg.world.octomap);
 
   return result;