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