Fix multithreading stepping in 3.12 and later (#1798)

* Fix multithreaded stepping to not have 'return' events when a thread is already suspended

* Update after removing blank line

* Remove unnecessary change for start method

tests/debug/session.py

@@ -867,6 +867,8 @@ class Session(object):
             assert len(expected_frames) <= len(frames)
             assert expected_frames == frames[0 : len(expected_frames)]
 
+        assert len(frames) > 0
+
         fid = frames[0]("id", int)
         return StopInfo(stopped, frames, tid, fid)