Fixes a mismatch between Cycles and EEVEE when rendering a
Vector Math node in Refract mode with a normal vector of 0,0,0.
This mismatch first appeared after 8650068f0c which changed the
behaviour in Cycles, but not EEVEE.
Pull Request: https://projects.blender.org/blender/blender/pulls/125644
be50c96c00