Participations: Filter out participations without activities.

lib/pleroma/conversation/participation.ex

@@ -79,5 +79,6 @@ defmodule Pleroma.Conversation.Participation do
         | last_activity_id: activity_id
       }
     end)
+    |> Enum.filter(& &1.last_activity_id)
   end
 end

test/conversation/participation_test.exs

@@ -86,4 +86,17 @@ defmodule Pleroma.Conversation.ParticipationTest do
 
     assert participation_one.last_activity_id == activity_three.id
   end
+
+  test "Doesn't die when the conversation gets empty" do
+    user = insert(:user)
+
+    {:ok, activity} = CommonAPI.post(user, %{"status" => ".", "visibility" => "direct"})
+    [participation] = Participation.for_user_with_last_activity_id(user)
+
+    assert participation.last_activity_id == activity.id
+
+    {:ok, _} = CommonAPI.delete(activity.id, user)
+
+    [] = Participation.for_user_with_last_activity_id(user)
+  end
 end