fixed support for moodle2 archives without user data

storagetype/moodle2.php

@@ -41,7 +41,12 @@ class archive_storagetype_moodle2 extends archive_storagetype_moodle {
                     throw new Exception("Can't load  $xml");
                 }
             } else {
-                throw new Exception("Can't load  $xml");
+                // users.xml is missing in backups without user information
+                if ($infotype == 'users') {
+                    $simple = new SimpleXMLElement('<users></users>');
+                } else {
+                    throw new Exception("Can't load  $xml");
+                }
             }
             /* Send only desired path in xml */
             if($info['path']){