Fix PHP 8.1 "strlen(null)" exception when trying to add empty user email address

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15377

Test Plan: Applied this change, afterwards repeated the steps to add a new email address on /settings/panel/email/ and left the "Email" field empty. This time, after selecting the "Save" button, the error message "Email is required." is displayed in the "New Address" overlay dialog.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15377

Differential Revision: https://we.phorge.it/D25211