prevent php8 warnung, fixes #4381

Closes #4381

Merge request studip/studip!3190

lib/meine_seminare_func.inc.php

@@ -129,7 +129,7 @@ function sort_groups($group_field, &$groups)
                 return (int)($a['gruppe'] - $b['gruppe']);
             } else {
                 if (Config::get()->IMPORTANT_SEMNUMBER) {
-                    return strnatcasecmp($a['sem_nr'], $b['sem_nr']);
+                    return strnatcasecmp($a['sem_nr'] ?? '', $b['sem_nr'] ?? '');
                 } else {
                     return strnatcmp($a['name'], $b['name']);
                 }