Cleanbanning a member who have left returns None

[Kitty-Therapist]

Describe the bug
When cleanbanning a member who have left the server, GearBot errors out and responds with "None."

Steps To Reproduce

1. Have a member to join the server to spam nonsense.
2. Ask them to leave.
3. Attempt to cleanban the member.
4. Observe

Expected behavior
I should have an error that explains why GearBot couldn't cleanban the user.

Screenshots
https://cdn.discordapp.com/attachments/281283303326089216/628622558303879198/unknown.png

Additional context
N/A.