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