Fix for ATLASRECTS-3787

This fix ensures that the MuonTruthDecorationAlg always adds the CaloEntryLayer_, MuonEntryLayer_, and MuonExtLayer_cov_extr variables to the truth particle.
The only difference is thus that an empty vector will, in rare cases, be added to truth particles: this will happen rarely enough that running the Tier0 tests shows no changes.
Output from the Tier0 tests is available at /afs/cern.ch/work/w/wleight/public/MuonSW/ATLASRECTS-3787/.
Note also that this only applies to MC.


Former-commit-id: 4eaa392faa0f9226571bb20ed147d99ea7b32c1e