OGP/TwitterCard: Add fallbacks in case the attachment key is nonexistent

lib/pleroma/web/metadata/opengraph.ex

@@ -121,4 +121,6 @@ defmodule Pleroma.Web.Metadata.Providers.OpenGraph do
       acc ++ rendered_tags
     end)
   end
+
+  defp build_attachments(_), do: []
 end

lib/pleroma/web/metadata/twitter_card.ex

@@ -116,6 +116,7 @@ defmodule Pleroma.Web.Metadata.Providers.TwitterCard do
       acc ++ rendered_tags
     end)
   end
+  defp build_attachments(_id, _object), do: []
 
   defp player_url(id) do
     Pleroma.Web.Router.Helpers.o_status_url(Pleroma.Web.Endpoint, :notice_player, id)