vtkRenderer::PickProp: Fix crash handling empty selection
This commit is a follow-up of 1eb083a8 (use hardwareselector to perform picking). It fixes a crash in vtkRenderer::PickProp happening when the selection is empty. This was discovered in 3D Slicer during the creation of widget:
vtkAbstractWidget::SetEnabled
-> this->WidgetRep->ComputeInteractionState(X, Y)
-> vtkAnnotationROIRepresentation2D::ComputeInteractionState
-> this->HandlePicker2D->Pick
-> crash
Edited by Jean-Christophe Fillion-Robin