Handle refracting object with index 0
Handle refracting object with index 0

The division by zero lead to direction scaled by infinity which in turn lead
to NaN coordinates for refracted ray head that was asserted to have length 1

Robustness is desired because the author of the scene may set the index to
zero but leave the optics transparent which would result in this to occure

biotty/rmg,biotty/rmg,biotty/rmg,biotty/rmg