Use \Drupal\user\UserInterface::TIMEZONE_DEFAULT to improve code readability

Problem/Motivation

In User::preSave() we do:

    $config = \Drupal::config('system.date');
    if ($config->get('timezone.user.configurable') && !$this->getTimeZone() && !$config->get('timezone.user.default')) {
      $this->set('timezone', $config->get('timezone.default'));
    }

Steps to reproduce

But $config->get('timezone.user.default') is not a boolean - it is a tri-state switch using the \Drupal\user\UserInterface::TIMEZONE_DEFAULT, \Drupal\user\UserInterface::TIMEZONE_EMPTY and \Drupal\user\UserInterface::TIMEZONE_SELECT constants so let's use them here.

Proposed resolution

Use the constant in the if

Remaining tasks

User interface changes

None

Introduced terminology

N/a

API changes

N/a

Data model changes

None

Release notes snippet

N/a