DCMathSegmentMaker - Do not create segments without measurements
This fixes warnings like
MuonEDMHelperSvc 9 5 WARNING Got segment without valid identifiers
MuGirlAlg_LRT.MuonInsid...MuonSegmentSelectionTool 9 5 WARNING Got invalid segment identifier
The segments do not have any measurements associated with them. So we kill them as early as possible.