MDL-27920 gradebook: fixes following integration review

grade/export/xml/index.php

@@ -58,8 +58,7 @@ if ($data = $mform->get_data()) {
     $export->process_form($data);
     $export->print_continue();
 
-    $require_user_idnumber = true; //skip users without idnumber as they cannot be identified when importing
-    $export->display_preview($require_user_idnumber);
+    $export->display_preview(true); //true == skip users without idnumber as they cannot be identified when importing
     echo $OUTPUT->container(get_string('useridnumberwarning','gradeexport_xml'), 'useridnumberwarning mdl-align');
 
     echo $OUTPUT->footer();

grade/lib.php

@@ -275,7 +275,7 @@ class graded_users_iterator {
     function _pop() {
         global $DB;
         if (empty($this->gradestack)) {
-            if (!$this->grades_rs) {
+            if (empty($this->grades_rs) || !$this->grades_rs->valid()) {
                 return null; // no grades present
             }