Check if group is set

The actual value is possible to be null,
thus leading to PHP notice without the check.

For PHP7.4

src/mibew/libs/groups.php

@@ -202,6 +202,10 @@ function get_group_email($group_id)
 {
     // Try to get group email
     $group = group_by_id($group_id);
+    if (empty($group)) {
+        return '';
+    }
+
     if ($group && !empty($group['vcemail'])) {
         return $group['vcemail'];
     }