Error occurs when going from ArcGIS Graphic to GeoJSON Feature and no "attributes" object is defined

Sample:
```javascript
// Pre:
// geometry is a valid ArcGIS Geometry object
// symbol is a valid ArcGIS Symbol object

var graphic = new esri.Graphic(geometry, symbol);
var feature = Terraformer.ArcGIS.parse(graphic.toJson());

// Results in error thrown by "eachPosition" method in terraformer.js
```
Adding an attribute fixes the problem:
```javascript
graphic.setAttributes({ "someAttr": "someValue" });
```