Allow for returning an empty dict if Nested object is None

Flask-RESTful only allows returning null if the object that is marshaled with Nested is None. This change adds a kwarg to allow returning an empty object instead (which seems like a better idea for the JSON object).
Example:
```
{
    "movie": {
        "id": 997, 
        "imdb_data": {
            "id": 816692, 
            "rating": "9.0", 
            "title": "Interstellar"
        }, 
        "show_end": 1416952800, 
        "show_start": 1415224800, 
        "title": "Interstellar", 
        "url": "http://planeta-kino.com.ua/lvov/movies/interstellar/"
    }
}
```
If there is no IMDB data for a movie then return an empty object:
```
{
    "movie": {
        "id": 1315, 
        "imdb_data": {}, 
        "show_end": null, 
        "show_start": 1417644000, 
        "title": "Trash", 
        "url": "http://planeta-kino.com.ua/lvov/movies/trash/"
    }
}
```
Inspired by this SA question: http://stackoverflow.com/q/26867357/695332