fix: don't compute offset if not needed (CURA-4103)
fix: don't compute offset if not needed (CURA-4103)

this helps withthe skin expansion distance if it the pre_shrink is zero

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