Fix arc patch PHP 8.1 strlen(null) error

Summary:
Fix arc patch PHP 8.1 strlen(null) error.

When doing an 'arc patch Dxxxx' against a diff on a PHP 8.1 Phorge server we get a strlen(null) error.
Updating the idx call to provide a default empty string resolves it.
Providing a default empty string to

Fixes T15571

Test Plan: arc patch Dxxxx

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

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

Maniphest Tasks: T15571

Differential Revision: https://we.phorge.it/D25364