Restless doesn't work with nullable foreign keys

I have a Category model with three fields: id, parent and title. Parent can be null.
I set up preparer like this:

```
preparer = FieldsPreparer(fields={
    'id': 'id',
    'parent': 'parent.id',
    'title': 'title',
})
```

If I request /api/categories/ I get an error:

```
'NoneType' object has no attribute 'id'
```
