pat::Muon best track embedding fix (7.0.X)

1) properly initialize embeddedTunePMuonBestTrack_ and embeddedTunePMuonBestTrack_
2) make the getter work also on existing 7_0_6 files (the bool was wrong, but the vector size was correct)
note: it doesn't need recompilation of any dependency, but it would be good to have this in a 7.0.X patch

@arizzi