Fix a PHP 8.1/8.2 deprecated call to strlen with a NULL argument

Summary:
This call was preventing notification servers configuration to be properly initialized.
Indeed, 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.

Fix T15596

Test Plan:
Sign in as an administrator, configure the notification server without filling admin path field,
you shouldn't get an invalid configuration error

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15596

Differential Revision: https://we.phorge.it/D25381