Protections to dtScalerInfo module to run online in MWGR

Validity of the getByToken from the dtScalerInfo module is checked as well as the size of the LumiScalersCollection before accesing information.
This fix affects only Online DQM (the module is not run in any offline DQM workflow) and is needed to properly run Online DT DQM during MWGR.