🐛 fix(chat.py): handle case when artifacts is empty to prevent potential error

The code now handles the case when the `artifacts` variable is empty by setting `input_keys_response` to an empty dictionary. This prevents potential errors that could occur when trying to build the `input_keys_response` object.

src/backend/langflow/api/v1/chat.py

@@ -160,6 +160,12 @@ async def stream_build(flow_id: str):
                 input_keys_response = build_input_keys_response(
                     langchain_object, artifacts
                 )
+            else:
+                input_keys_response = {
+                    "input_keys": {},
+                    "memory_keys": [],
+                    "handle_keys": [],
+                }
             yield str(StreamData(event="message", data=input_keys_response))
 
             chat_manager.set_cache(flow_id, langchain_object)