Like posted above, they will go empty unless they have non-revs to fill them. Yes the non-revs will have to list/pay for F or J, but even if they don't they will usually be moved into the cabin at boarding for free if the gate agent feels like doing it.
I agree with letting them go empty though, why give it away to revenue passengers who paid little for their tickets and have them sit next to someone who paid $5000?