From old forum:
Question: Show open tickets that have all assignments closed
Answer: As the report was made a long time ago and I would find it a little difficult to explain in writing, I'm providing you with images that I hope you can figure out and apply to your report. I was happy to find the report and found that is still works on the latest HEAT version we are running. I got help making this report a very long time ago from a master report writer at FrontRange.