Fix PHP notices upon enabling and using the module

[boobaa]

• [ ] Undefined offset: 0 in Drupal\password_enhancements\EventSubscriber\NavigationLock->onKernelRequestLockNavigation() (line 115 of modules/custom/password_enhancements/src/EventSubscriber/NavigationLock.php).
• [ ] Undefined offset: 0 in Drupal\password_enhancements\EventSubscriber\InitSubscriber->checkRequiredPasswordChange() (line 121 of modules/custom/password_enhancements/src/EventSubscriber/InitSubscriber.php).
• [ ] Undefined offset: 0 in Drupal\password_enhancements\PasswordChecker->isExpired() (line 77 of modules/custom/password_enhancements/src/PasswordChecker.php).

[boobaa]

This has been fixed by #19 already.