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