Narrow down getPredicate() return type

rdfInterface\DatesetInterface::getPredicate() and rdfInterface\DatesetNodeInterface::getPredicate() return type narrowed down to rdfInterface\NamedNodeInterface|null.

While technically this is a backward-incompatible change and thus marks major version bump, there is no known RDF extension nor implementation which would depend on the getPredicate() method returning anything else than NamedNodeIterface|null.