fix to be able to run on MiniAODs without errors

but without much sensible output so far
please include check for the existence of the parameter like in https://github.com/cms-sw/cmssw/blob/CMSSW_7_3_X/RecoTauTag/RecoTau/plugins/PFRecoTauDiscriminationByIsolation.cc#L53

There might be more config files calling this module, so one should be able to keep old behaviour in case the parameter is not defined.