R1:94d45de2f7dd

Fix PHP 8.1 "ltrim(null)" exception which blocks adding additional user email address

Summary:
Since PHP 8.1, passing a null string to `ltrim(string $string)` is deprecated.

Thus we make sure that `$request->getStr('email')` does not return null as default.

Closes T15376

Test Plan: Applied this change, afterwards repeated the steps to add a new email address on `/settings/panel/email/`. This time, it's possible to close the "Verification Email Sent" and the page `/settings/panel/email/` renders and…