don't remove blank node from graph.getStatement and keep blank node id

When a post is made to create an annotation, no rdf:about is set yet because the resource is created by the server.
graph.getStatement should still return the statements. I created a getAllStatement function in the NXAS branch but I believe it should be the default behavior.

The blank node id should also be kept to be able to "redo" the graph.

I added org.nuxeo.ecm.platform.relations.jena.GraphLoadingTest with commented test that I would expect to pass after the change