[JTC] Sending an empty traj topic overrides the current action goal

[christophfroehlich]

Sending an empty trajectory message from the topic interface (not the action interface) will override the current action goal and not abort the action.

          I think it will be overridden, but it should not be. The explanation is correct, but I  don't think it is desirable.

Originally posted by @destogl in https://github.com/ros-controls/ros2_controllers/pull/566#discussion_r1163167321