Prevent floating point exceptions in Marching Cubes.
Normalizing this vector caused a divide-by-zero in the VTK unit test VTK::AcceleratorsVTKmCxx-TestVTKMMarchingCubes.
Normalizing this vector caused a divide-by-zero in the VTK unit test VTK::AcceleratorsVTKmCxx-TestVTKMMarchingCubes.