DataAssertions requires an Attributes member

[GregoryAveryWeir]

At https://github.com/nilportugues/php-json-api/blob/f72538b770f71c49fd75e4637d782edba1f5cbb9/src/Server/Data/DataAssertions.php#L38 , each request is validated to ensure it has an attributes member. However, the JSON API spec says resource objects MAY contain an attributes member. There are cases where you might want to leave it out of a request, as when you don't want to modify any attributes but do have meta properties to PATCH.