[Bugfix][Hardware][CPU] Fix CPU model input for decode (#9044)

Author: Isotr0py

vllm/worker/cpu_model_runner.py

@@ -133,7 +133,7 @@ def build(self) -> ModelInputForCPU:
             (input_tokens, input_positions,
              attn_metadata) = self._prepare_decode(
                  self.seq_group_metadata_list)
-            seq_lens = []
+            seq_lens = None
 
         return self.model_input_cls(
             input_tokens=input_tokens,