whisker can't deal with null values

The whisker rendering fail if i try to do something like:
```js
whisker.add(myNode, {
  text : null
 });
``` 
```html
<div class="myNode">{{item.text}}</div>
```

It throws an exception: Uncaught TypeError: Cannot read property 'undefined' of null .

To fix this I do:
```js
whisker.add(myNode, {
  text : null || ''
 });
``` 