R1:8310591523f5

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…