[fix]解决单测test_encode报错问题

vllm/engine/llm_engine.py

@@ -989,6 +989,7 @@ class LLMEngine:
                 output = [outputs_by_sequence_group[0][i]]
 
             # tree style speculative decoding may generate empty output in first step
+            if outputs and isinstance(output[0], SamplerOutput):
                 samples = [o.samples[0] for o in output]
                 valid_samples = [
                     sample for sample in samples