Fix RichMedia negative cache entries

The negative cache entry was a nil value, but that is an expected response when the cache is missing an entry so it didn't work as intended.

lib/pleroma/web/rich_media/backfill.ex

@@ -64,5 +64,5 @@ defp stream_update(%{"activity_id" => activity_id}) do
   defp warm_cache(key, val), do: @cachex.put(:rich_media_cache, key, val)
 
   defp negative_cache(key, ttl \\ :timer.minutes(15)),
-    do: @cachex.put(:rich_media_cache, key, nil, ttl: ttl)
+    do: @cachex.put(:rich_media_cache, key, :error, ttl: ttl)
 end

test/pleroma/web/rich_media/backfill_test.exs

@@ -0,0 +1,26 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.RichMedia.BackfillTest do
+  use Pleroma.DataCase
+
+  alias Pleroma.Web.RichMedia.Backfill
+  alias Pleroma.Web.RichMedia.Card
+
+  import Mox
+
+  setup_all do: clear_config([:rich_media, :enabled], true)
+
+  test "sets a negative cache entry for an error" do
+    url = "https://bad.example.com/"
+    url_hash = Card.url_to_hash(url)
+
+    Tesla.Mock.mock(fn %{url: ^url} -> :error end)
+
+    Pleroma.CachexMock
+    |> expect(:put, fn :rich_media_cache, ^url_hash, :error, ttl: _ -> {:ok, true} end)
+
+    Backfill.run(%{"url" => url})
+  end
+end