Fix PHP 8.1 "strlen(null)" exception which blocks rendering the "Manage Menu" 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.
However, the parameter `$item_id` is a numeric value when populated.
Thus replace `strlen()` with `phutil_nonempty_scalar()` as `phutil_nonempty_string()` could break a Conduit API consumer sending a numeric value.

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

Closes T15317

Test Plan: Applied this change and `/home/menu/configure/` 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: T15317

Differential Revision: https://we.phorge.it/D25166

src/applications/search/engine/PhabricatorProfileMenuEngine.php

@@ -135,7 +135,7 @@ abstract class PhabricatorProfileMenuEngine extends Phobject {
     if ($is_view) {
       $selected_item = $this->selectViewItem($view_list, $item_id);
     } else {
-      if (!strlen($item_id)) {
+      if (!phutil_nonempty_scalar($item_id)) {
         $item_id = self::ITEM_MANAGE;
       }
       $selected_item = $this->selectEditItem($view_list, $item_id);