- open system settings > account details > user manager
- type an email address to your user account
- click on "Apply" button, type your password and press enter/return to confirm
- close the User Manager and reopen it
- clear the email address field
The empty email is saved.
Can't go in 5.18 because you added a new string. That part would need to go on master. So either this whole thing goes on master, or the bug gets fixed in 5.18 and the new messagebox goes on master.