Issue #3240361 by andypost, alexpott:...

Issue #3240361 by andypost, alexpott: \Drupal\user\Entity\User::checkExistingPassword() causes deprecations on PHP 8.1

core/modules/user/src/Entity/User.php

@@ -404,7 +404,9 @@ public function setExistingPassword($password) {
    * {@inheritdoc}
    */
   public function checkExistingPassword(UserInterface $account_unchanged) {
-    return strlen($this->get('pass')->existing) > 0 && \Drupal::service('password')->check(trim($this->get('pass')->existing), $account_unchanged->getPassword());
+    $existing = $this->get('pass')->existing;
+    return $existing !== NULL && strlen($existing) > 0 &&
+      \Drupal::service('password')->check(trim($existing), $account_unchanged->getPassword());
   }
 
   /**