The voids are a non-issue as long as something is covering it and structurally supporting it, like the metal from an unmodified knob, or a screw-on knob that is glued-on. If you think about it, most strong structures have intentional voids- either to improve rigidity for its mass, or reduce weight. On top of that, the end of the handle at the knob is one of the less-stressed parts.