[sciurius]

Yes, the latter. I have enabled Settings->Library Settings->Show Setlist Duration since that is a great addition.
When editing a setlist, the (current) duration is displayed and it is okay to be 0:00 while editing.
But on the Setlist display I think it has no added value to display [0:00] for setlists that have no songs assocated or contain only songs without duration.

I would even consider some way of signaling that not all songs of a setlist have a valid duration, since that would make the total duration inaccurate.