Fix #133768: Grease Pencil: Select strokes with active material fails

Caused by fd83be0fa5. Following `if()` condition doesn't seem useful. Entire
curve range is returned when no material is locked and active material
index is ignored.

Pull Request: https://projects.blender.org/blender/blender/pulls/133798

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

@@ -1027,9 +1027,6 @@ IndexMask retrieve_editable_strokes_by_material(Object &object,
 
   /* Get all the editable material indices */
   VectorSet<int> locked_material_indices = get_locked_material_indices(object);
-  if (locked_material_indices.is_empty()) {
-    return curves_range;
-  }
 
   const bke::AttributeAccessor attributes = curves.attributes();