Fix DifferentialCommitMessageField renderFieldValue PHP 8.1 strlen(null) error

Summary:
arc diff throws strlen(null) error from DifferentialCommitMessageField renderFieldValue when calling a Phorge server running PHP 8.1

Add unit test, which required a new DifferentialTestCommitMessageField class so as to be able to test the abstract DifferentialCommitMessageField class methods.

Fixes T15530

Test Plan:
Make a change in a git repo with remote a Phorge server running PHP 8.1
Run:

arc diff

See exception thrown as per T15530

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

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

Maniphest Tasks: T15530

Differential Revision: https://we.phorge.it/D25334