Fix HarbormasterBuildUnitMessage PHP 8.1 strlen(null) error

Summary:
Fix HarbormasterBuildUnitMessage PHP 8.1 strlen(null) error.

Fixes T15572

Test Plan: Do an 'arc diff' to a PHP 8.1 Phorge server when doing so will trigger unit tests.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15572

Differential Revision: https://we.phorge.it/D25365