nil.usages is undefined when parsing RDF itself. #205
Description
When I parse the RDF ontology itself with jsonld.promises.fromRDF(nquads, {format: 'application/nquads'}) it causes an error on line 3237:
F:\wamp-new\www\dacore\modules\core\node_modules\jsonld\js\jsonld.js:3236
for(var i = 0; i < nil.usages.length; ++i) {
^
TypeError: Cannot read property 'length' of undefined
at Processor.fromRDF (F:\wamp-new\www\dacore\modules\core\node_modules\jsonld\js\jsonld.js:3236:34)
at fromRDF (F:\wamp-new\www\dacore\modules\core\node_modules\jsonld\js\jsonld.js:908:23)
at F:\wamp-new\www\dacore\modules\core\node_modules\jsonld\js\jsonld.js:903:7
at _combinedTickCallback (internal/process/next_tick.js:73:7)
at process._tickCallback (internal/process/next_tick.js:104:9)
This seems to be because RDF_NIL occurs in the graph (because the RDF ontolgy defines it) but is not used as an object anywhere in the RDF ontology.]