Fix behavior if publisher_uri is not available

[seitenbau-govdata]

If no URI for the publisher is available in the serialized RDF graph currently always the URL from the organization is used. This can lead to same URI values for different publisher names if there is a publisher name in the RDF graph. To avoid this we should not use the URL from the organization if there is a publisher name but no publisher URI in the serialized RDF graph.

[amercader]

Looks good @seitenbau-govdata !

[seitenbau-govdata]

Thanks a lot @amercader! 😃