Generation: past kv can be None (#31051)

* fix

* better

src/transformers/generation/utils.py

@@ -1394,7 +1394,7 @@ class GenerationMixin:
             return model_kwargs
 
         past_length = 0
-        if "past_key_values" in model_kwargs:
+        if model_kwargs.get("past_key_values") is not None:
             if isinstance(model_kwargs["past_key_values"], Cache):
                 past_length = model_kwargs["past_key_values"].get_seq_length()
             else: