Fix tab complete in php 8

Summary: Ref T15064

Test Plan: run arc shell-complete; arc shell-complete arc shell; Should suggest completions, not crash.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: mainframe98, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25350