Always serialize _embedded

An API should be consistent. If when we have elements in a collection, we get a response like

```json
{
    "_embedded": {
        "users": [
            {
                "username": "bob"
            },
            {
                "username": "joe"
            }
        ]
    }
}
```

and then when there are no users found we get a response like

```json
{
}
```

it is not consistent and forces our clients to handle the special case of not having `_embedded.users` in the response.

Instead, what our clients reasonably expect is a response like

```json
{
    "_embedded": {
        "users": [
        ]
    }
}
```