Update srdl2.pl to allow queries for robots with capabilities

A query like:

 cap_available_on_robot('http://ias.cs.tum.edu/kb/srdl2-cap.owl#GraspingCapability', R).

currently causes a failure because the call "rdfs_individual_of(Robot, RobotClass)" fails with a "not initialized" Exception if no robot is provided in the query. switching the order of the calls solves the problem for me