Don't break on empty mentions.

lib/pleroma/web/ostatus/handlers/note_handler.ex

@@ -55,8 +55,9 @@ defmodule Pleroma.Web.OStatus.NoteHandler do
   end
 
   def get_mentions(entry) do
-    get_people_mentions(entry)
-    ++ get_collection_mentions(entry)
+    (get_people_mentions(entry)
+      ++ get_collection_mentions(entry))
+    |> Enum.filter(&(&1))
   end
 
   def make_to_list(actor, mentions) do

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -270,7 +270,11 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPI do
     user = User.get_cached_by_ap_id(actor)
     # mentioned_users = Repo.all(from user in User, where: user.ap_id in ^activity.data["to"])
     mentioned_users = Enum.map(activity.data["to"] || [], fn (ap_id) ->
-      User.get_cached_by_ap_id(ap_id)
+      if ap_id do
+        User.get_cached_by_ap_id(ap_id)
+      else
+        nil
+      end
     end)
     |> Enum.filter(&(&1))