Honestly, it's very kind of you to even ask this question! But since they are finishing up before dinner starts, you don't need to arrange dinner for them. Usually vendors won't like to hang around just to hang around anyway after they finish as it's not like being at a wedding socially, so they will usually just pack up and be on their way anyhow.
If you're concerned, maybe just mention in an email to the photographer that just to confirm, since they're finished before dinner you haven't arranged for a meal, and is that alright. I'm 99% sure they'll say absolutely fine. :-)