Fix rqt_jtc with num_cmd<dof

rqt_joint_trajectory_controller/rqt_joint_trajectory_controller/joint_trajectory_controller.py

@@ -457,12 +457,9 @@ def _joint_widgets(self):  # TODO: Cache instead of compute every time?
 
 
 def _jtc_joint_names(jtc_info):
-    # NOTE: We assume that there is at least one hardware interface that
-    # claims resources (there should be), and the resource list is fetched
-    # from the first available interface
 
     joint_names = []
-    for interface in jtc_info.claimed_interfaces:
+    for interface in jtc_info.required_state_interfaces:
         name = interface.split("/")[-2]
         if name not in joint_names:
             joint_names.append(name)