[BUG]: Failing to account for get_user returning None.

### Bot Version

v3.10.2

### How are you hosting Modmail?

Other

### Error Logs

https://paste.pythondiscord.com/nepupiwime.txt?noredirect

### Screenshots

_No response_

### Additional Information

This happens when the bot is trying to close pending threads but the cache doesn't contain the users who scheduled the closing. 