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)
Just visit /admin/customize/site_texts and search for “that’s sad”. You can change it to whatever you want.
EDIT: Though it looks like “that’s sad” has been changed in core. Looks like it’s just this now:
bottom:
latest: "There are no more latest topics."
posted: "There are no more posted topics."
read: "There are no more read topics."
new: "There are no more new topics."
unread: "There are no more unread topics."
unseen: "There are no more unseen topics."
category: "There are no more %{category} topics."
tag: "There are no more %{tag} topics."
top: "There are no more top topics."
bookmarks: "There are no more bookmarked topics."