Simplify check for list frame.

Author: gkellogg

lib/frame.js

@@ -234,9 +234,9 @@ api.frame = (state, subjects, frame, parent, property = null) => {
 
         // recurse into list
         if(graphTypes.isList(o)) {
-          const subframe = (frame[prop] && types.isObject(frame[prop][0]) ?
-            (frame[prop][0]['@list'] || _createImplicitFrame(flags)) :
-            _createImplicitFrame(flags));
+          const subframe = (frame[prop] && frame[prop][0] && frame[prop][0]['@list']) ?
+            frame[prop][0]['@list'] :
+            _createImplicitFrame(flags);
 
           // add empty list
           const list = {'@list': []};