fix: only adjust member count if guild is found

[AstreaTSS]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change
• CI change
• Other: [Replace with a description]

Description

I don't quite have an idea on how it happened, but according to a message in the NAFF Discord server, sometimes, when a member is removed from a guild, the guild they were removed from isn't in the cache and causes the processor to fail.

This PR causes the processor to not error, at the very least.

Changes

• Only change guild.member_count if guild isn't None in MemberEvents.

Checklist

• I've formatted my code with Black
• I've added docstrings to everything I've touched
• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x
• I've tested my changes
• I've added tests for my code - if applicable
• I've updated the documentation - if applicable