fix: log warning on missing default currency instead of error

As the problem can be handled, it is not worth an error.

src/QUI/ERP/Currency/Handler.php

@@ -200,7 +200,7 @@ public static function getDefaultCurrency(): ?Currency
                     $Config->getValue('currency', 'defaultCurrency')
                 );
             } catch (QUI\Exception $Exception) {
-                QUI\System\Log::addError('Default currency is missing');
+                QUI\System\Log::addWarning('Default currency is missing');
 
                 try {
                     self::$Default = self::getCurrency('EUR');