Fix crash in DTDCSByLumiSummary when MonitorElements are missing

This change properly protects the module from the case where the
MonitorElements it wants from DQMStore are not available.
This fixes a crash seen in the threaded IB RelVals.
Merge pull request #4975 from Dr15Jones/fixCrashInDTDCSByLumiSummary

Fix crash in DTDCSByLumiSummary when MonitorElements are missing