Fix output queue handling in OmniStage to return None if queue is closed during shutdown

Signed-off-by: tzhouam <tzhouam@connect.ust.hk>

vllm_omni/entrypoints/omni_stage.py

@@ -689,8 +689,10 @@ class OmniStage:
         Returns:
             Result dictionary if available, None otherwise. Result contains
             request_id, engine_outputs (or engine_outputs_shm), and metrics.
+            Returns None if output queue is already closed (e.g. during shutdown).
         """
-        assert self._out_q is not None
+        if self._out_q is None:
+            return None
         try:
             return self._out_q.get_nowait()
         except Exception: