prevent quoted nulls ("null") when to_json is called on nil relationship

Fixes the case where calling something like:

`resource.to_json(:relationships => { :nested_resource => { :only => [:id] } })`

When the nested resource returns nil, would return invalid JSON like…

```
{
  "id":1,
  "nested_resouce":"null"
}
```

Rather than valid JSON like…

```
{
  "id":1,
  "nested_resouce":null
}
```

(Note the quotes around null)