fix: correct filtering of assignments by staffing count

src/Form/AssignmentFilterType.php

@@ -67,6 +67,7 @@ class AssignmentFilterType extends AbstractType
                 'choices' => [
                     'voll besetzt' => Assignment::STATUS_STAFFED,
                     'teilweise besetzt' => Assignment::STATUS_PARTLY_STAFFED,
+                    'unbesetzt' => Assignment::STATUS_UNSTAFFED,
                     'mit Bewerbungen' => Assignment::STATUS_STAFFING,
                 ],
             ]);