JointPositionController topics don't show up when listing topics with ign topic -l.

To reproduce, run the nao jointpositioncontroller demo:

ign gazebo -v 4 https://fuel.ignitionrobotics.org/1.0/OpenRobotics/worlds/NAO%20joint%20control

and in a new terminal, run

ign topic -l

I've briefly looked at the code for the JointPositionController.cc, but it looks similar to other plugins that show up in the topic list.

ign gazebo version: 6.11.0