Joint trajectory controller: Send empty msg through action interface

[adolfo-rt]

I just discovered that sending an empty message through the action interface does not have the same behavior as PR2's JointTrajectoryActionController:

• PR2: Reject goal because joint list (empty) doesn't match those of the controller.
• ros_controllers: Accept goal and have the same effect as sending an empty msg trough the topic interface: cancel all queued trajectory segments.

In the ros_controllers case, an unexpected behavior of its current implementation is that after accepting the empty goal, it remains active. Not very intuitive. I'm opting for maintaining the behavior of the PR2 controller, maybe someone even depends on that.