Fix PHP 8.1 "strlen(null)" exception for "/bin/aphlict notify" without parameter

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.

This patch also fixes another similar strlen() occurrence in the same source file.

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/aphlict/management/PhabricatorAphlictManagementNotifyWorkflow.php:29]

Test Plan: Run ../phorge/bin/aphlict notify: Get no strlen() error anymore but only expected output Usage Exception: Specify a user to notify with "--user".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25590