Fix PHP 8.1 "strlen(null)" exception which blocks rendering MFA Provider page

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 T15386

Test Plan: Applied this change, afterwards page "Mobile Phone App (TOTP)" at `/auth/mfa/4/` renders in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15386

Differential Revision: https://we.phorge.it/D25218

src/applications/auth/controller/mfa/PhabricatorAuthFactorProviderViewController.php

@@ -83,7 +83,7 @@ final class PhabricatorAuthFactorProviderViewController
 
 
     $custom_enroll = $provider->getEnrollMessage();
-    if (strlen($custom_enroll)) {
+    if (phutil_nonempty_string($custom_enroll)) {
       $view->addSectionHeader(
         pht('Custom Enroll Message'),
         PHUIPropertyListView::ICON_SUMMARY);