[Bugfix] Fix output token length check logic (#16419)

Signed-off-by: look <eeslook@163.com>

benchmarks/benchmark_serving.py

@@ -156,7 +156,7 @@ def calculate_metrics(
         if outputs[i].success:
             output_len = outputs[i].output_tokens
 
-            if output_len is None:
+            if not output_len:
                 # We use the tokenizer to count the number of output tokens
                 # for some serving backends instead of looking at
                 # len(outputs[i].itl) since multiple output tokens may be