add JECpayload protection in PhysicsTools/PatUtils/interface/ShiftedJetProducerT.h

Following a request from @vadler, a protection is added in ShiftedJetProducerT.h in order to run on simulation with PhysicsTools/PatAlgos/test/patTuple_metUncertainties_cfg.py  even if residual jet corrections are missing (as in the auto:run2_mc GT configuration)