Fix URL filtering to match old server
This MR 'fixes' the server-side URL filtering to match the old server, by only applying the filter-preference when a urltype is given. Compare this to https://git.ligo.org/computing/ligo-data-replicator/-/blob/9ff125b/datafind-server/DataFindServer/LDRDataFindServerEngine.py#L184-L186.