Fix the saver of empty URI returning local:local

[syphax-bouazzouni]

For the case of 4store, when we insert the following statement:
<http://data.bioontology.org/ontologies/ANAEETHES/submissions/1> <http://omv.ontoware.org/2005/05/ontology#useImports> <> .

is, in reality, saving the following
<http://data.bioontology.org/ontologies/ANAEETHES/submissions/1> <http://omv.ontoware.org/2005/05/ontology#useImports> <local:local> .

Even worse inserting <>(empty URIs) is not supported by Alegrograph (causing the failure of a lot of our tests)