downgrade undeserved error log for empty JointState.position

as per the JointState message doc, the ``position`` member must be
either empty OR have the same size as the ``name`` member.

In case it is empty, the message is valid, but will still warn
we won't use it.

src/joint_state_listener.cpp

@@ -86,7 +86,11 @@ void JointStateListener::callbackFixedJoint(const ros::TimerEvent& e)
 void JointStateListener::callbackJointState(const JointStateConstPtr& state)
 {
   if (state->name.size() != state->position.size()){
-    ROS_ERROR("Robot state publisher received an invalid joint state vector");
+    if (state->position.empty()){
+      ROS_WARN_THROTTLE(300, "Robot state publisher ignored a JointState message whose position member was empty");
+    } else {
+      ROS_ERROR("Robot state publisher ignored an invalid JointState message");
+    }
     return;
   }