KdTree: add a function to test if it is empty
KdTree: add a function to test if it is empty

Former-commit-id: 96696759aa3df2c6fa71bfb51d85f6f2d83089f6
FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa