Don't break on empty mentions.

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

@@ -55,8 +55,9 @@ def get_collection_mentions(entry) 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 @@ defp activity_to_status(activity, opts) 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) ->
+      if ap_id do
         User.get_cached_by_ap_id(ap_id)
+      else
+        nil
+      end
     end)
     |> Enum.filter(&(&1))
 

test/web/ostatus/ostatus_test.exs

@@ -346,4 +346,11 @@ test "insert or update a user from given data" do
 
     assert {:ok, %User{}} = OStatus.insert_or_update_user(data)
   end
+
+  test "it doesn't add nil in the do field" do
+    incoming = File.read!("test/fixtures/nil_mention_entry.xml")
+    {:ok, [activity]} = OStatus.handle_incoming(incoming)
+
+    assert activity.data["to"] == ["http://localhost:4001/users/atarifrosch@social.stopwatchingus-heidelberg.de/followers", "https://www.w3.org/ns/activitystreams#Public"]
+  end
 end