GPv3: Fix: Avoid the creation of new objects for empty material slots when separating by material

This PR fix the creation of new objects for empty material slots

Pull Request: https://projects.blender.org/blender/blender/pulls/119062

source/blender/editors/grease_pencil/intern/grease_pencil_edit.cc

@@ -2035,6 +2035,10 @@ static bool grease_pencil_separate_material(bContext &C,
 
   /* Create a new object for each material. */
   for (const int mat_i : IndexRange(object_src.totcol).drop_front(1)) {
+    if (!BKE_object_material_slot_used(&object_src, mat_i + 1)) {
+      continue;
+    }
+
     Object *object_dst = duplicate_grease_pencil_object(
         &bmain, &scene, &view_layer, &base_prev, grease_pencil_src);