Fix error in outbox when no activities exist

lib/clacks_web/controllers/outbox_controller.ex

@@ -50,13 +50,22 @@ defmodule ClacksWeb.OutboxController do
   end
 
   defp outbox_page(conn, pagination_params, activities) do
+    next =
+      case activities do
+        [] ->
+          # todo: is nil the right thing here?
+          nil
+
+        _ ->
           last_id = List.last(activities).id
+          current_url(conn, %{max_id: last_id})
+      end
 
     %{
       "type" => "OrderedColletionPage",
       "partOf" => current_url(conn, %{}),
       "id" => current_url(conn, pagination_params),
-      "next" => current_url(conn, %{max_id: last_id}),
+      "next" => next,
       "orderedItems" => Enum.map(activities, & &1.data)
     }
   end