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 lists the new email address.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15376
Differential Revision: https://we.phorge.it/D25210
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