early out when meshgroup is empty (CURA-349)
early out when meshgroup is empty (CURA-349)

totalretribution/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,totalretribution/CuraEngine,pratikshashroff/pcura,ROBO3D/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,pratikshashroff/pcura,pratikshashroff/pcura,totalretribution/CuraEngine,Ultimaker/CuraEngine