MDL-81634 core: Fix all implicitly defined nullables

Note: This does not impact third-party libraries.

lib/classes/persistent.php

@@ -49,7 +49,7 @@ abstract class persistent {
      * @param int $id If set, this is the id of an existing record, used to load the data.
      * @param stdClass $record If set will be passed to {@link self::from_record()}.
      */
-    public function __construct($id = 0, stdClass $record = null) {
+    public function __construct($id = 0, ?stdClass $record = null) {
         global $CFG;
 
         if ($id > 0) {
@@ -975,7 +975,7 @@ abstract class persistent {
      * @param array $params
      * @return bool
      */
-    public static function record_exists_select($select, array $params = null) {
+    public static function record_exists_select($select, ?array $params = null) {
         global $DB;
         return $DB->record_exists_select(static::TABLE, $select, $params);
     }