Prevent task session delivery NOT NULL crashes

* fix: coerce null endpoint_url when delivering task result to a session

* fix: also coerce null model so the session insert satisfies NOT NULL

* test: cover task session delivery on an empty database

src/task_scheduler.py

@@ -1300,8 +1300,8 @@ class TaskScheduler:
             sess = DbSession(
                 id=session_id,
                 name=f"[Task] {task.name}",
-                endpoint_url=endpoint_url,
-                model=model_name,
+                endpoint_url=endpoint_url or "",
+                model=model_name or "",
                 owner=task.owner,
                 created_at=datetime.utcnow(),
                 updated_at=datetime.utcnow(),