Don't ingest empty availability windows from LoveFilm

 - OnDemandLocationGenerator now returns an Optional<OnDemandLocation> to reflect the fact that it may not always generate an OnDemandLocation for a given Item.
 - LoveFilmOnDemandLocationGenerator now does not check for null start/end dates in the Item's Policy, but rather looks for the presence/absence of an Encoding, to determine whether to generate an OnDemandLocation.
 - added a test for the above.