Remove extra newline from N-Triples output (#1999)

Remove the extra newline generated by the N-Triples serializer and add
a unit test to verify that N-Triples output does not end in two
consecutive newlines.

Closes #1998

rdflib/plugins/serializers/nt.py

@@ -39,7 +39,6 @@ def serialize(
 
         for triple in self.store:
             stream.write(_nt_row(triple).encode())
-        stream.write("\n".encode())
 
 
 class NT11Serializer(NTSerializer):

test/test_issues/test_issue1998.py

@@ -0,0 +1,15 @@
+"""Test that N-Triples formats do not have double newlines at the end
+of the output.
+
+https://github.com/RDFLib/rdflib/issues/1998
+"""
+import rdflib
+
+
+def test_1998():
+    g = rdflib.Graph()
+    bob = rdflib.URIRef("http://example.org/people/Bob")
+    g.add((bob, rdflib.RDF.type, rdflib.FOAF.Person))
+    for nt_format in ("nt", "nt11"):
+        output = g.serialize(format=nt_format)
+        assert not output.endswith("\n\n")