Minimum password length field not required → NULL passed to t() causes TypeError

Problem/Motivation

On the configuration page at /admin/config/people/passwords, the “Minimum password length” field is not required.
If the field is left empty, better_passwords_after_build() calls t() with a NULL value:

$element['#description'] .= '<br/>' . t(
  'Passwords must be at least @num characters.',
  ['@num' => $config->get('length')] // <--- $config->get('length') can be NULL
);

This triggers the same core error as in:
https://www.drupal.org/project/drupal/issues/3554974
"TypeError: Drupal\Component\Utility\Html::escape(): Argument #1 ($text) must be of type string, null given, called in core/lib/Drupal/Component/Render/FormattableMarkup.php on line 238"

Steps to reproduce

• Install and enable Better Passwords.
• Go to /admin/config/people/passwords.
• Leave Minimum password length empty.
• Save configuration.
• Go to /admin/people/create and you get a WSOD

Proposed resolution

Wrap the code in a conditional check, the same way it is already done for the strength setting directly below it:

if (!empty($config->get('length'))) {
  $element['#description'] .= '<br/>' . t(
    'Passwords must be at least @num characters.',
    ['@num' => $config->get('length')]
  );
}