"Administration role" setting in User Account settings form defaults to first, randomly available user role

Problem

1. Install Drupal with Minimal profile.
2. Create a new user role.
3. Go to /admin/config/people/accounts
4. Bogus default value:

   

Cause

1. The default value in user.settings is admin_role: ''
2. user_admin_settings() contains:
   

     $roles = user_role_names();
     unset($roles[DRUPAL_ANONYMOUS_RID]);
     unset($roles[DRUPAL_AUTHENTICATED_RID]);
     $roles[0] = t('disabled');
   
     $form['admin_role']['user_admin_role'] = array(
       '#type' => 'select',
       '#title' => t('Administrator role'),
       '#default_value' => $config->get('admin_role'),
       '#options' => $roles,
   

3. The default value is 0 (integer), whereas the actual default value is '' (empty string).

Proposed solution

1. Replace this:
   

     $roles[0] = t('disabled');
   

   With this:

       '#empty_value' => '',
       '#empty_option' => t('- Disabled -'),