Fix PHP 8.1 "strlen(null)" exceptions which block rendering Auth's Customize Messages 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 T15325

Test Plan: Applied these three changes and all seven subpages (e.g. `/auth/message/user.edit.username/`) finally rendered 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: T15325

Differential Revision: https://we.phorge.it/D25172

src/applications/auth/controller/message/PhabricatorAuthMessageViewController.php

@@ -103,7 +103,7 @@ final class PhabricatorAuthMessageViewController
       ->setViewer($viewer);
 
     $full_description = $message_type->getFullDescription();
-    if (strlen($full_description)) {
+    if (phutil_nonempty_string($full_description)) {
       $view->addTextContent(new PHUIRemarkupView($viewer, $full_description));
     } else {
       $short_description = $message_type->getShortDescription();
@@ -111,7 +111,7 @@ final class PhabricatorAuthMessageViewController
     }
 
     $message_text = $message->getMessageText();
-    if (strlen($message_text)) {
+    if (phutil_nonempty_string($message_text)) {
       $view->addSectionHeader(
         pht('Message Preview'),
         PHUIPropertyListView::ICON_SUMMARY);
@@ -120,7 +120,7 @@ final class PhabricatorAuthMessageViewController
     }
 
     $default_text = $message_type->getDefaultMessageText();
-    if (strlen($default_text)) {
+    if (phutil_nonempty_string($default_text)) {
       $view->addSectionHeader(
         pht('Default Message'),
         PHUIPropertyListView::ICON_SUMMARY);