Allow postLS1 customisation for reduced geometries 

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

tests:
- python runs,
- checked with prints that customisation is completed with full geometry