Fix preamble-support

Summary:
Phutil is not yet loaded during preamble, so we can't use it.

This change fixes a regression introduced here, fixing PHP 8.1 support:

R1:96ae4ba13acb: PHP 8.1: fixes for strlen() not accepting NULL anymore, part 2

Ref T15064

Test Plan: Included test script.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25114