Sort using

Hi, 
I am using the `sort` attribute on `published_at` (date type) for documents. 
But it looks like if the document doesn't have `published_at` attribute, the sorting won't display the document: 

```html

"sort" :[
        { "published_at" : "desc"} 
 ]

``` 

It should sort the documents à the end even the attributes is missing no ?