fix: handle nullable values in responsible attribute

Author: lukas-staab

app/Models/Legacy/Project.php

@@ -99,7 +99,7 @@ class Project extends Model
     protected function responsible(): Attribute
     {
         return Attribute::make(
-            get: fn (string $value) => empty($value) || str_contains($value, '@') ? $value : $value.'@'.config('stufis.mail_domain'),
+            get: fn (?string $value) => empty($value) || str_contains($value, '@') ? $value : $value.'@'.config('stufis.mail_domain'),
             set: fn (string $value) => empty($value) || str_contains($value, '@') ? $value : $value.'@'.config('stufis.mail_domain'),
         );
     }