Encoder fails to encode empty data if field set is specified

Test snippet

``` php
$actual = $endcoder->encode([], null, null, new EncodingParameters(null, [
    // include only these attributes and links
    'authors' => [Author::ATTRIBUTE_FIRST_NAME, Author::LINK_COMMENTS],
]));
```

> PHP Fatal error:  Call to a member function getIncludePaths() on a non-object in /json-api/src/Encoder/Encoder.php on line 196
