Serializer: remove empty lists on toRdf()

[juliapampus]

The toRdf method removes null objects. This is nice, as we then have a clean rdf string without unnecessary information. With the new version, these empty lists for all list attributes bloat up the ids rdf string. Can you maybe change the toRdf method to handle empty lists as null?

[sebbader]

Hi Julia, looks like something changed in a Jackson library underneath. We are looking into it.