Fix bench_serving fail when zero warmup requests (sgl-project#5574)

Author: fzyzcjy

python/sglang/bench_serving.py

@@ -690,7 +690,6 @@ def sample_random_requests(
     dataset_path: str,
     random_sample: bool = True,
 ) -> List[Tuple[str, int, int]]:
-
     input_lens = np.random.randint(
         max(int(input_len * range_ratio), 1),
         input_len + 1,
@@ -1025,7 +1024,9 @@ async def limited_request_func(request_func_input, pbar):
     warmup_outputs = await asyncio.gather(*warmup_tasks)
 
     # Check if at least one warmup request succeeded
-    if not any(output.success for output in warmup_outputs):
+    if args.warmup_requests > 0 and not any(
+        output.success for output in warmup_outputs
+    ):
         raise ValueError(
             "Warmup failed - Please make sure benchmark arguments "
             f"are correctly specified. Error: {warmup_outputs[0].error}"