fix: correctly check if translation of unit exists

(cherry picked from commit 21a31f8f)

Co-authored-by: Patrick Müller <p.mueller@pcsg.de>

src/QUI/ERP/Accounting/Article.php

@@ -567,7 +567,7 @@ public function getQuantityUnit(QUI\Locale $Locale = null): string
             if (isset($options['entries'][$unitId])) {
                 $titles = $options['entries'][$unitId]['title'];
 
-                if ($titles[$current]) {
+                if (!empty($titles[$current])) {
                     return $titles[$current];
                 }
             }