Fix PHP warning.

main/inc/lib/course.lib.php

@@ -5508,8 +5508,11 @@ class CourseManager
     {
         $grouplist = array();
         $userlist = array();
+
         foreach ($to as $to_item) {
+            if (!empty($to_item)) {
                 list($type, $id) = explode(':', $to_item);
+
                 switch ($type) {
                     case 'GROUP':
                         $grouplist[] = intval($id);
@@ -5519,9 +5522,11 @@ class CourseManager
                         break;
                 }
             }
+        }
 
         $send_to['groups'] = $grouplist;
         $send_to['users'] = $userlist;
+
         return $send_to;
     }