TPC QC allow proper init of propagator to avoid error message in FST - #1436

Detectors/TPC/qc/src/Tracks.cxx:185:          LOGP(error, "o2::base::Propagator not properly initialized, MatLUT ({}) and / or Field ({}) missing, will not fill DCA histograms", (void*)propagator->getMatLUT(), propagator->hasMagFieldSet());