Fix PHP 8.1 DifferentialBranchField getBranchDescription strlen(null) error

Summary:
Fix PHP 8.1 strlen(null) error in DifferentialBranchField getBranchDescription()

Fixes T15531

Test Plan:
Simply viewing a diff (eg https://my.phorge.site/D1234) on a PHP 8.1 server generated the error fixed by this diff.

Also created a unit test.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15531

Differential Revision: https://we.phorge.it/D25335