In Node Map Generation, skip objects where @id is null.

Author: gkellogg

lib/nodeMap.js

@@ -186,6 +186,11 @@ api.createNodeMap = (input, graphs, graph, issuer, name, list) => {
 
       // handle embedded subject or subject reference
       if(graphTypes.isSubject(o) || graphTypes.isSubjectReference(o)) {
+        // skip null @id
+        if('@id' in o && !o['@id']) {
+          continue;
+        }
+
         // relabel blank node @id
         const id = graphTypes.isBlankNode(o) ?
           issuer.getId(o['@id']) : o['@id'];