Fix PHP 8 exit status cannot be null error in PhutilArgumentParser

Summary:
Passing null as the first parameter ($status) to exit has been deprecated
and leads to a PHP error.

Closes T15990

Test Plan:
Run any bin scripts with PHP 8.4.
bin/arc version should finish successfully without PHP errors.

Reviewers: Blessed Committers, O1 Blessed Committers, valerio.bozzolan

Reviewed By: Blessed Committers, O1 Blessed Committers, valerio.bozzolan

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

Tags: PHP 8 support

Maniphest Tasks: T15990

Differential Revision: https://we.phorge.it/D25869