fix test: isEmpty() requires that robot_state be diff

Fixes #116. This was probably broken by https://github.com/ros-planning/moveit_core/commit/103a268a819f7f9e2c3adb929f5948ab62e44f7f in which isEmpty(RobotState) began to require is_diff to be true -- of course, the default value is false.

@isucan -- I chose to fix the test here -- but perhaps the definition of isEmpty() is logically wrong instead?