Fix the unset argument in calling llama model (#2714)

When using the llama.cpp together with agent like
zero-shot-react-description, the missing branch will cause the parameter
`stop` left empty, resulting in unexpected output format from the model.

This patch fixes that issue.

langchain/llms/llamacpp.py

@@ -177,6 +177,8 @@ class LlamaCpp(LLM):
             raise ValueError("`stop` found in both the input and default params.")
         elif self.stop:
             params["stop_sequences"] = self.stop
+        elif stop:
+            params["stop_sequences"] = stop
         else:
             params["stop_sequences"] = []