avoid python errors when running cmsDriver with options

--customise SLHCUpgradeSimulations/Configuration/postLS1Customs.customisePostLS1

in combination with geomegtries that don't include CSCs, e.g.

--geometry HCal
--geometry ECALHCAL
small fix
Merge pull request #5005 from lveldere/postLS1Customs-convenience-fix

Allow postLS1 customisation for reduced geometries