Fix bug when no posts are in the db.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -157,7 +157,7 @@ defp restrict_favorited_by(query, _), do: query
 
   # Only search through last 100_000 activities by default
   defp restrict_recent(query, _) do
-    since = Repo.aggregate(Activity, :max, :id) - 100_000
+    since = (Repo.aggregate(Activity, :max, :id) || 0) - 100_000
 
     from activity in query,
       where: activity.id > ^since

test/web/activity_pub/activity_pub_test.exs

@@ -82,6 +82,8 @@ test "retrieves activities that have a given context" do
 
   describe "public fetch activities" do
     test "retrieves public activities" do
+      activities = ActivityPub.fetch_public_activities
+
       %{public: public} = ActivityBuilder.public_and_non_public
 
       activities = ActivityPub.fetch_public_activities