Fixes NaN in Vector Displacement node caused by the normalization of 0, 0, 0 vectors. This fixes both visual rendering issues and an "illegal address" error on the GPU. The "illegal address" error came from the Light Tree Sampling code not handling the NaN normals well, leading to weird code paths being taken, eventually leading to a kernel_assert and a user facing illegal address error. Pull Request: https://projects.blender.org/blender/blender/pulls/111294