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