[bench] Fix benchmark/serve.py to ignore unavailable results (#22382)

Signed-off-by: Linkun <github@lkchen.net>

vllm/benchmarks/serve.py

@@ -665,7 +665,7 @@ def save_to_pytorch_benchmark_format(args: argparse.Namespace,
     pt_records = convert_to_pytorch_benchmark_format(
         args=args,
         metrics={k: [results[k]]
-                 for k in metrics},
+                 for k in metrics if k in results},
         extra_info={
             k: results[k]
             for k in results if k not in metrics and k not in ignored_metrics