Fix: NaN in vector math node in refract mode

Fix a NaN that can occur in the vector math node when set to
refract mode with a 'normal' input of length zero.

Pull Request: https://projects.blender.org/blender/blender/pulls/125374

intern/cycles/kernel/svm/math_util.h

@@ -37,7 +37,7 @@ ccl_device void svm_vector_math(ccl_private float *value,
       *vector = reflect(a, b);
       break;
     case NODE_VECTOR_MATH_REFRACT:
-      *vector = refract(a, normalize(b), param1);
+      *vector = refract(a, safe_normalize(b), param1);
       break;
     case NODE_VECTOR_MATH_FACEFORWARD:
       *vector = faceforward(a, b, c);