You can leave it alone and it will eventually grow branches. If you want to encourage branching you will need to top it. You don't have to take very much off. Just take off the terminal bud at the very top and leave whatever leaves there are below. The part that needs to be taken off is only about 1/2".

The three leaves are at the very top, so even cutting off 1/2 inch will leave me without any leaves. Would that be okay? I'd want to wait until spring, right?

By cutting back you will encourage new growth that will send out new leaves. Figs only support leaves on new growth. The harder you cut, the more new growth that will be produced. If you look at your Negronne, you can probably see where the old wood stops and the new (this year's) wood starts. The old wood doesn't have any leaves.
Waiting until spring is usually a good idea. You are working with nature. Are you going to let the figs go dormant?

I hadn't planned on letting them go dorment, since I have the greenhouse and it is heated to 60 degrees or so. Is there an advantage to letting them go dorment?

I guess I should add: I also hadn't planned on letting them go dorment since I wasn't sure what the temp range is for dormency vs. death. We had a fig tree when I lived in the deep south as a kid, but I never saw any when I lived up north. Now I live kinda in between, in KY, so I'm not sure how far below freezing figs can survive.

I wouldn't see any problem with a tree that had a 15 inch trunk before it branched.