Don't combine infill layers if infill is empty
Don't combine infill layers if infill is empty

If the infill density is 0 and it has to combine layers, it would try to access layers below the first layer that don't exist. Instead it will now not combine at all, saving some processing power too.

Contributes to issue Cura-285.
ROBO3D/CuraEngine,Ultimaker/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,totalretribution/CuraEngine,markwal/CuraEngine,totalretribution/CuraEngine,markwal/CuraEngine,alephobjects/CuraEngine,pratikshashroff/pcura,markwal/CuraEngine,pratikshashroff/pcura,ROBO3D/CuraEngine,alephobjects/CuraEngine,pratikshashroff/pcura,totalretribution/CuraEngine