MDL-76362 various: Avoid passing nulls to functions that don't allow nulls

PHP 8.1 is more strict on the parameter type. Functions such as trim(), strlen(), str_replace(), etc
show notice when null is passed as an argument

course/format/classes/base.php

@@ -608,7 +608,7 @@ abstract class base {
         $course = $this->get_course();
         try {
             $sectionpreferences = (array) json_decode(
-                get_user_preferences('coursesectionspreferences_' . $course->id, null, $USER->id)
+                get_user_preferences('coursesectionspreferences_' . $course->id, null, $USER->id) ?? ''
             );
             if (empty($sectionpreferences)) {
                 $sectionpreferences = [];