Get list of change sets with no work item associated ?

I don't know any APIs that can filter on the work item association of a change set. The best option I can think of is to get a complete list of all change sets and filter them after the list is obtained.

For example, you can POST to /ccm/oslc-scm/changeSet with request headers "X-Jazz-CSRF-Prevent: <current JSESSIONID>" and "X-Method-Override:REPORT", and payload '{"propertyRequest":{"jazz_scm:reasons":{}}}', then in the response JSON, look for '"jazz_scm:reasons": []'.

POST without the specified payload, or a GET will not return any "jazz_scm:reasons" entries.