Fix AFK system returning an error if the message guild isn't already indexed in the user_presence database

[notsniped]

It would return this exception in stdout for absolutely no reason (if someone were to message in a server where no one is AFK)

Ignoring exception in on_message
Traceback (most recent call last):
  File "/opt/virtualenvs/python3/lib/python3.8/site-packages/discord/client.py", line 343, in _run_event
    await coro(*args, **kwargs)
  File "main.py", line 104, in on_message
    for x in user_presence[str(ctx.guild.id)].keys(): uList.append(x)
KeyError: '873417861589262367'

It's kinda annoying seeing this exception flooding the verbose log when there isn't even anything wrong with the code or the bot.