N-Triples serializer adds extra newline

[tcmitchell]

The N-Triples serializer includes an extra newline at the end. This newline is unnecessary.

This impacts our ability to sort the N-Triples lines in order to produce a sorted N-Triples format that is suitable for diff and version control.

Here is code to demonstrate the issue. Note the pair of \n characters at the end of the output for both "nt" and "nt11" formats.

>>> import rdflib
>>> g = rdflib.Graph()
>>> bob = rdflib.URIRef("http://example.org/people/Bob")
>>> g.add((bob, rdflib.RDF.type, rdflib.FOAF.Person))
<Graph identifier=N9146103a52ed4e7b88e761835d34ec04 (<class 'rdflib.graph.Graph'>)>
>>> g.serialize(format='nt')
'<http://example.org/people/Bob> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Person> .\n\n'
>>> g.serialize(format='nt11')
'<http://example.org/people/Bob> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Person> .\n\n'
>>>