Queue's empty list should return 200 and an object

The queue's endpoint is not consistent with the rest of the API nor with
the API specs. It currently returns 204 and no response when there are
no queue's in the system. Instead, it should return 200 OK and an object
with an empty `queues` list.

Closes-bug: #1379220
Change-Id: Ib3af5ef3829cc82f3e96bb5401f71b7599f5c5c8