Fix: Don't crash if a message has no author

[smichel17]

When setting the status message, the bot looks for the prior message, to update it if it is found. To do this, it filters the message list by author id. However, sometimes a message does not have an author (unsure when; maybe when the author has left the server?), and the bot crashes.

This slipped by for a long time because the types don't mark the author as nullable (although the api reference does).

Honestly I'm a bit surprised it didn't happen before now, since #117 didn't change this filter

[smichel17]

After running into a similar error in a different place (:wastebasket: reactions to remove embeds), I believe this is a result of the message being partial, so it could be fixed by if (msg.partial) await msg.fetch(). This might be worth doing at some point, since I think the same issue may result in the bot updating an older status message sometimes. Not going to do that today, though.