refs #9048 Trying to fix Coverity noise in ConvToMDSelector

at https://scan6.coverity.com:8443/reports.htm#v29240/p10083/fileInstanceId=11025316&defectInstanceId=3876567&mergedDefectId=528357

do not understand what Coverity is complaining about -- default occurs if neither EventWS nor Matrix2D ws is provided (e.g. Table) Rewritten in a way which hopefully is more clear for Coverity