fix runtime error in AutoGen agent

[cpacker]

Fix #65 (comment)

return asyncio.run(
File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run
return loop.run_until_complete(main)
File "/usr/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
return future.result()
File "/home/ana/.local/lib/python3.10/site-packages/memgpt/autogen/memgpt_agent.py", line 148, in _a_generate_reply_for_user_message
user_message = new_messages[0]
IndexError: list index out of range

[cpacker]

is None a safe early exit object to return here?

the expected type AFAIK is a message, eg

{
  "role": "assistant",
  "content": "assistant's message"
}

[vivi]

lgtm 😼