Sculpt: Avoid setting vertex normal to the zero vector

It is possible for the simple vertex normal calculated during Paint
BVH normal updates to have a length of 0 if the connected faces cancel
each other out. This is a generally unlikely scenario, but will almost
always lead to further corruption of the mesh with the new normal.

If we detect this case, set the vertex normal to (0, 0, 1)

Pull Request: https://projects.blender.org/blender/blender/pulls/146004

source/blender/blenkernel/intern/pbvh.cc

@@ -1120,7 +1120,11 @@ static void normals_calc_verts_simple(const GroupedSpan<int> vert_to_face_map,
     for (const int face : vert_to_face_map[vert]) {
       normal += face_normals[face];
     }
-    vert_normals[vert] = math::normalize(normal);
+    float length;
+    vert_normals[vert] = math::normalize_and_get_length(normal, length);
+    if (length == 0.0f) {
+      vert_normals[vert] = float3(0, 0, 1);
+    }
   }
 }