Revert "Use [] instead of nil"

This reverts commit 42268b0981.

lib/pleroma/web/ostatus/activity_representer.ex

@@ -271,7 +271,8 @@ def to_simple_form(%{data: %{"type" => "Undo"}} = activity, user, with_author) d
            []}
         ] ++ mentions ++ author
 
-      _ -> []
+      _ ->
+        nil
     end
   end