When using dotNotation, return null if the path cannot be expanded

This is a small change to have dotNotation return null when the full path cannot be evaluated (instead of throwing an exception, as it does now).  Our use case for this is Backbone-relational with Rivet.js for databinding.  For example, we have:

```
 %a{data-bind-href: 'item.id | conversation_link', data-bind-text: 'item.assignee.name'}
```

Not all items are assigned, so the current implementation breaks.  This change just leaves the text blank.