Omit empty attributes

[f3ath]

Issue

According to the standard, the attributes member of the resource must be an object. If the attributes list is empty, json_encode treats it as an array, producing invalid output like this:

{"data":{"type":"posts","id":"1","attributes":[]}}

This behavior breaks some clients which expect attributes to be an object, not an array. In the same time, attributes is an optional member.

Proposed solution

Omit empty attributes in the resulting json.

[tobyzerner]

Beaut, thanks :)