Make empty() const
Make empty() const

No need to overwrite it.

Contributes to issue CURA-3491.

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