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."