The "Lightset 0" Group is a special instance and will always exist and have the id of "0" as specified in the Hue Api documentation. Due to this internal group being maintained by the bridge internally, it will not return an array of light ids like any of the other types of Groups.
However, on my hue bridge, when I issue a GET request for /api/newdeveloper/groups/0 in CLIP, the result does include the list of lights:
The readme says:
However, on my hue bridge, when I issue a
GET
request for /api/newdeveloper/groups/0 in CLIP, the result does include the list of lights: