Fix: Crash adding rest positions to empty mesh

The position attribute might not exist when the mesh is empty, which may
be another problem, but there is no need to count on it here.

source/blender/blenkernel/intern/DerivedMesh.cc

@@ -747,7 +747,7 @@ static void mesh_calc_modifiers(struct Depsgraph *depsgraph,
     MutableAttributeAccessor attributes = mesh_final->attributes_for_write();
     SpanAttributeWriter<float3> rest_positions =
         attributes.lookup_or_add_for_write_only_span<float3>("rest_position", ATTR_DOMAIN_POINT);
-    if (rest_positions) {
+    if (rest_positions && attributes.domain_size(ATTR_DOMAIN_POINT) > 0) {
       attributes.lookup<float3>("position").materialize(rest_positions.span);
       rest_positions.finish();
     }