Bad SPARQLStore handling of empty literals

[bcroq]

When an empty literal is returned to a SPARQLStore, it becomes an rdflib.term.Literal(u'None') instead of an rdflib.term.Literal(u''):

# ./fuseki-server --update --mem /empty_literal

import rdflib
from rdflib.plugins.stores.sparqlstore import SPARQLUpdateStore

store = SPARQLUpdateStore('http://fuseki.til.io:3030/empty_literal/query',
                          'http://fuseki.til.io:3030/empty_literal/update')

graph = rdflib.Graph(store, 'http://example.com/')

graph.add((rdflib.URIRef('http://example.com/s'), 
           rdflib.URIRef('http://example.com/p'), 
           rdflib.Literal('')))

o = tuple(graph)[0][2]
assert o == rdflib.Literal(''), repr(o)

raises an AssertionError: rdflib.term.Literal(u'None')

The problem seems to be in rdflib.plugins.stores.sparqlstore:CastToTerm:

import io
from xml.etree import ElementTree

import rdflib
from rdflib.plugins.stores.sparqlstore import CastToTerm

s = """<ns0:literal xmlns:ns0="http://www.w3.org/2005/sparql-results#" />"""
node = ElementTree.parse(io.BytesIO(s)).getroot()
term = CastToTerm(node)
assert term  == rdflib.Literal(''), repr(term)

raises an AssertionError: rdflib.term.Literal(u'None')

[joernhees]

@gjhiggins could you review the changes in #459 ? There is some weird if False in the segment i patched

[bcroq]

@joernhees I confirm that the changes in #459 fix the problem.