Instead of catching a null reference for non-existing mail accounts, it seems
to be easier to only get the mail acccount when we actually want to do
something with it. Since we are only intersted in the change of the display
name, the retreival of the user account has been moved to that if-branch.
This also seems more like a Nextcloud bug, since it fails on errors of hooks.
My IMAP and SMTP servers expect email address style user names, and from what I see elsewhere I would suspect these default settings to be more robust across different setups.