fix(prefs): guard config-file storage against null $registry global

[ralflang]

Summary

Horde_Core_Prefs_Storage_Configuration::get() reaches out to global $registry to load prefs.php defaults. Under modern PSR-15 request paths (e.g. the admin REST API — Horde\Horde\Admin\UserController::get() → IdentityService::getAll() → SqlPrefsService::getValue()), $GLOBALS['registry'] is never populated and the class fatals with Call to a member function loadConfigFile() on null.

Add a guard: when $registry isn't a Horde_Registry instance, return $scope_ob unchanged. Same graceful fallback shape the existing try/catch uses when prefs.php doesn't exist. Downstream storage drivers (SQL etc.) still supply real user preferences.

Reproducer against a locally-installed Horde:

hordectl query user admin
→ ❌ 500 Internal Server Error (before)
→ 200 with user payload (after)

Config-file defaults are best-effort atop user prefs. When we can't reach the registry to load them, "skip defaults, fall through to real storage" is the right degrade.

Reported while dogfooding hordectl webserver-config in horde/hordectl PR #17 CI. Surfaces on any admin REST API call that touches IdentityService.