[Perf] Optimize requests abort (#32211)

Signed-off-by: yewentao256 <zhyanwentao@126.com>

vllm/v1/engine/async_llm.py

@@ -525,6 +525,7 @@ class AsyncLLM(EngineClient):
                             await asyncio.sleep(0)
 
                         # 3) Abort any reqs that finished due to stop strings.
+                        if processed_outputs.reqs_to_abort:
                             await engine_core.abort_requests_async(
                                 processed_outputs.reqs_to_abort
                             )