MDL-27395 Improved detection of the course grade_category on restore

Note this was applied months ago to 21 and master (MDL-28152),
but due to one miss-interpretation by this integrator (stronk7),
I missed to backport it to 20_STABLE where the problem was
also reproducible.

So, here it is, the fix is trivial, safe and proved to work.

backup/moodle2/restore_stepslib.php

@@ -236,6 +236,7 @@ protected function process_grade_category($data) {
             //get the already created course level grade category
             $category = new stdclass();
             $category->courseid = $this->get_courseid();
+            $category->parent = null;
 
             $coursecategory = $DB->get_record('grade_categories', (array)$category);
             if (!empty($coursecategory)) {