MRF: anti followbot: gracefully handle accounts without a display name

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

@@ -23,15 +23,21 @@ 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.
     nick_score =
       nickname
       |> String.downcase()
       |> score_nickname()
 
+    # displayname will either be a binary string or nil, if a displayname isn't set.
     name_score =
-      displayname
-      |> String.downcase()
-      |> score_displayname()
+      if is_binary(displayname) do
+        displayname
+        |> String.downcase()
+        |> score_displayname()
+      else
+        0.0
+      end
 
     nick_score + name_score
   end