I think there are two scenarios and they need different messages:
latest tab is empty (I think @justin suggestion of “You’re all caught up! Ready to [start a new conversation]?” is a good one here.
New category is empty (or site is brand new, etc). This one needs more of an exhortation to create some topics so people have something to talk about, and real actual topic examples of what the category is for (other than the category description)