Use [] instead of nil

lib/pleroma/web/ostatus/activity_representer.ex

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