Just make a new activity. Instructions on how to do that are on the wiki, and it has been asked many times on the forum before.
But running empty on a line such as this would not at all be efficient; as subsidies on public transport are related to in-service running, closing a trip to passengers isn't only costly in missing out on passenger fares, but also in exploitation subsidies. And as long as you are running past the stations anyway on route to St Enoch, it isn't much of an effort to take passengers along.
You can see that in Amsterdam too; although the tram depot Havenstraat is not next to a tram end-of-line, the trams depart start taking passengers on board as soon as they reach the first intermediate stop on the line they'll be running on.