partners/mistralai: Fix KeyError in Vertex AI stream (#28624)

- **Description:** Streaming response from Mistral model using Vertex AI
raises KeyError when trying to access `choices` key, that the last chunk
doesn't have. The fix is to access the key safely using `get()`.
  - **Issue:** #27886
  - **Dependencies:**
  - **Twitter handle:**

libs/partners/mistralai/langchain_mistralai/chat_models.py

@@ -595,7 +595,7 @@ def _stream(
         for chunk in self.completion_with_retry(
             messages=message_dicts, run_manager=run_manager, **params
         ):
-            if len(chunk["choices"]) == 0:
+            if len(chunk.get("choices", [])) == 0:
                 continue
             new_chunk = _convert_chunk_to_message_chunk(chunk, default_chunk_class)
             # make future chunks same type as first chunk
@@ -621,7 +621,7 @@ async def _astream(
         async for chunk in await acompletion_with_retry(
             self, messages=message_dicts, run_manager=run_manager, **params
         ):
-            if len(chunk["choices"]) == 0:
+            if len(chunk.get("choices", [])) == 0:
                 continue
             new_chunk = _convert_chunk_to_message_chunk(chunk, default_chunk_class)
             # make future chunks same type as first chunk