fix: dont process skin if no roofing layers will be generated (CURA-3940)
fix: dont process skin if no roofing layers will be generated (CURA-3940)

ROBO3D/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine