handle missing finish_reason (#7296)

In some cases, the OpenAI response is missing the `finish_reason`
attribute. It seems to happen when using Ada or Babbage and
`stream=true`, but I can't always reproduce it. This change just
gracefully handles the missing key.

langchain/llms/openai.py

@@ -48,9 +48,9 @@ def update_token_usage(
 def _update_response(response: Dict[str, Any], stream_response: Dict[str, Any]) -> None:
     """Update response from the stream response."""
     response["choices"][0]["text"] += stream_response["choices"][0]["text"]
-    response["choices"][0]["finish_reason"] = stream_response["choices"][0][
-        "finish_reason"
-    ]
+    response["choices"][0]["finish_reason"] = stream_response["choices"][0].get(
+        "finish_reason", None
+    )
     response["choices"][0]["logprobs"] = stream_response["choices"][0]["logprobs"]