BACKEND: Fix KeyError: 'client_msg_id'

[Iapetus-11]

backend-1   | ERROR:slack_bolt.AsyncApp:Failed to run listener function (error: 'client_msg_id')
backend-1   | Traceback (most recent call last):
backend-1   |   File "/slack-emoji-leaderboard/backend/.venv/lib/python3.10/site-packages/slack_bolt/listener/asyncio_runner.py", line 118, in run_ack_function_asynchronously
backend-1   |     await listener.run_ack_function(request=request, response=response)
backend-1   |   File "/slack-emoji-leaderboard/backend/.venv/lib/python3.10/site-packages/slack_bolt/listener/async_listener.py", line 119, in run_ack_function
backend-1   |     return await self.ack_function(
backend-1   |   File "/slack-emoji-leaderboard/backend/src/app.py", line 171, in app_handle_message
backend-1   |     id=event["client_msg_id"],
backend-1   | KeyError: 'client_msg_id'
backend-1   | ERROR:slack_bolt.AsyncApp:Faile