don't output empty collections

This is a PR to prevent a HAL response like this:
```json
{
    "key": "value",
    "_embedded": {
        "some-embedded-key": []
    }
}
```

As _embedded is defined as OPTIONAL in [draft-kelly-json-hal-06](https://tools.ietf.org/html/draft-kelly-json-hal-06#section-4.1.2) it doesn't make any sense in my opinion to pollute the response with empty collections.

Feedback and opinions are very welcome.

Thank you.