Fix AntiFollowbotPolicy when trying to follow a relay

lib/pleroma/web/activity_pub/mrf/anti_followbot_policy.ex

@@ -25,11 +25,15 @@ defmodule Pleroma.Web.ActivityPub.MRF.AntiFollowbotPolicy do
   defp score_displayname(_), do: 0.0
 
   defp determine_if_followbot(%User{nickname: nickname, name: displayname}) do
-    # nickname will always be a binary string because it's generated by Pleroma.
+    # nickname will be a binary string except when following a relay
     nick_score =
-      nickname
-      |> String.downcase()
-      |> score_nickname()
+      if is_binary(nickname) do
+        nickname
+        |> String.downcase()
+        |> score_nickname()
+      else
+        0.0
+      end
 
     # displayname will either be a binary string or nil, if a displayname isn't set.
     name_score =