BSE: fix undo_stack_to_descriptor() on dummy undo stacks
BSE: fix undo_stack_to_descriptor() on dummy undo stacks

Dummy undo stacks lack project association, so cannot produce proper object
descriptors. Dummy stacks are used by undoable methods of orphan objects
(usually during destruction), their steps never get executed, so we can
simply resort to a static descriptor which will never be used.

tim-janik/beast,swesterfeld/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast,GNOME/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,GNOME/beast,swesterfeld/beast,swesterfeld/beast,GNOME/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast