Fix the timeline of users with a nil pinned_objects field.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -641,7 +641,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
       |> Map.put(:type, ["Create", "Announce"])
       |> Map.put(:user, reading_user)
       |> Map.put(:actor_id, user.ap_id)
-      |> Map.put(:pinned_object_ids, Map.keys(user.pinned_objects))
+      |> Map.put(:pinned_object_ids, Map.keys(user.pinned_objects || %{}))
 
     params =
       if User.blocks?(reading_user, user) do

test/pleroma/web/mastodon_api/controllers/account_controller_test.exs

@@ -267,6 +267,17 @@ defmodule Pleroma.Web.MastodonAPI.AccountControllerTest do
   describe "user timelines" do
     setup do: oauth_access(["read:statuses"])
 
+    test "works with a user who has an empty pinned objects field", %{conn: conn} do
+      user = insert(:user, pinned_objects: nil)
+
+      assert resp =
+               conn
+               |> get("/api/v1/accounts/#{user.id}/statuses")
+               |> json_response_and_validate_schema(200)
+
+      assert [] == resp
+    end
+
     test "works with announces that are just addressed to public", %{conn: conn} do
       user = insert(:user, ap_id: "https://honktest/u/test", local: false)
       other_user = insert(:user)