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 we will still warn
we won't use it.

src/joint_state_listener.cpp

@@ -88,7 +88,16 @@ 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()){
+      const int throttleSeconds = 300;
+      ROS_WARN_THROTTLE(throttleSeconds,
+          "Robot state publisher ignored a JointState message about joint(s) "
+          "\"%s\"(,...) whose position member was empty. This message will "
+          "not reappear for %d seconds.", state->name[0].c_str(),
+          throttleSeconds);
+    } else {
+      ROS_ERROR("Robot state publisher ignored an invalid JointState message");
+    }
     return;
   }