MDL-38345 Prevent undefined CFG leading to DB error.

It seems that MDL-32975, no matter a version bump
was done, leads to the $CFG->navsortmycoursessort
setting undefined sometimes. And that ends with an error
visiting the /my page. This just ensures a safe default
if the setting is undefined/empty.

lib/navigationlib.php

@@ -2584,6 +2584,10 @@ class global_navigation extends navigation_node {
     protected function load_courses_enrolled() {
         global $CFG, $DB;
         $sortorder = 'visible DESC';
+        // Prevent undefined $CFG->navsortmycoursessort errors.
+        if (empty($CFG->navsortmycoursessort)) {
+            $CFG->navsortmycoursessort = 'sortorder';
+        }
         // Append the chosen sortorder.
         $sortorder = $sortorder . ',' . $CFG->navsortmycoursessort . ' ASC';
         $courses = enrol_get_my_courses(null, $sortorder);