Fix DifferentialDiff getFieldValuesForConduit PHP 8.1 strlen(null) errors

Summary:
Fix DifferentialDiff getFieldValuesForConduit PHP 8.1 strlen(null) errors by replacing the strlen() calls with phutil_nonempty_string().

Also needed to update DifferentialDiffTestCase.php to being a PhabricatorTestCase, as it was throwing an exception prior to any code changes -

EXCEPTION (Exception): Trying to read configuration "policy.locked" before configuration has been initialized.

Fixes T15529

Test Plan: arc diff

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

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

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

Maniphest Tasks: T15529

Differential Revision: https://we.phorge.it/D25333