Check if nil.usages exists.

Fixes fromRdf with input triple having a nil subject.

Author: davidlehn

CHANGELOG.md

@@ -1,5 +1,8 @@
 # jsonld ChangeLog
 
+### Fixed
+- Fix fromRdf with input triple having a nil subject.
+
 ## 0.5.17 - 2018-01-25
 
 ### Changed

lib/fromRdf.js

@@ -184,6 +184,9 @@ api.fromRDF = async (
 
     // iterate backwards through each RDF list
     const nil = graphObject[RDF_NIL];
+    if(!nil.usages) {
+      continue;
+    }
     for(let usage of nil.usages) {
       let node = usage.node;
       let property = usage.property;