MDL-76362 core: Use empty default string when getting prefs

The json_decode function does not accept a null, which is the
traditional default for get_user_preferences. By passing a default of
am empty string we avoid issues in PHP 8.1.

course/format/classes/base.php

@@ -618,7 +618,7 @@ public function get_sections_preferences_by_preference(): array {
         $course = $this->get_course();
         try {
             $sectionpreferences = (array) json_decode(
-                get_user_preferences('coursesectionspreferences_' . $course->id, null, $USER->id) ?? ''
+                get_user_preferences("coursesectionspreferences_{$course->id}", '', $USER->id)
             );
             if (empty($sectionpreferences)) {
                 $sectionpreferences = [];

lib/tablelib.php

@@ -573,7 +573,7 @@ public static function get_sort_for_table($uniqueid) {
         global $SESSION;
         if (isset($SESSION->flextable[$uniqueid])) {
             $prefs = $SESSION->flextable[$uniqueid];
-        } else if (!$prefs = json_decode(get_user_preferences('flextable_' . $uniqueid), true)) {
+        } else if (!$prefs = json_decode(get_user_preferences("flextable_{$uniqueid}", ''), true)) {
             return '';
         }
 
@@ -1463,7 +1463,7 @@ protected function initialise_table_preferences(): void {
 
         // Load any existing user preferences.
         if ($this->persistent) {
-            $this->prefs = json_decode(get_user_preferences('flextable_' . $this->uniqueid) ?? '', true);
+            $this->prefs = json_decode(get_user_preferences("flextable_{$this->uniqueid}", ''), true);
             $oldprefs = $this->prefs;
         } else if (isset($SESSION->flextable[$this->uniqueid])) {
             $this->prefs = $SESSION->flextable[$this->uniqueid];
@@ -2346,4 +2346,3 @@ public function finish_document() {
         exit();
     }
 }
-