Create new client API degradation

Summary

After upgrade to 3.3.1 the creation of new client stopped working with the following error: ISPConfig SoapFault(data_processing_error): usertheme_snytax_incorrect

Steps to reproduce

1. Use the api to create a new client without populating the 'usertheme' field like so:

      $res = $this->call('client_add',
        Array('session_id' => $this->session_id,
          'reseller_id' => 0,
          'params' =>
            Array(
              'username' => substr($fullusername, 0, 25),
              'customer_no' => $fullusername,
              'contact_name' => $fullusername,
              'email' => '***masked***',
              'password' => $password,
              'web_php_options' => array('mod', 'php-fpm'),
              'ssh_chroot' => 'jailkit',
              'country' => 'BG',
              'language' => 'en',
              'template_master' => $template['template_id'],
              'limit_web_domain' => 1,
              'limit_web_quota' => $template['limit_web_quota'],
              'limit_database' => 1,
              'limit_cron' => 0,
              'limit_cron_type' => 'chrooted',
              'limit_cron_frequency' => 1,
            )));

$this->call is just a wrapper for the soap call

This code has not been changes since 2019 and it was working fine

Correct behaviour

It should have continue to work after the upgrade

Environment

Server OS + version: (Debian 12)
ISPConfig version: (3.3.1)

Software version of the related software:

Server version: Apache/2.4.66 (Debian)
Server built:   2025-12-05T18:54:44
PHP 8.1.34 (cli) (built: Feb 13 2026 15:51:52) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.34, Copyright (c) Zend Technologies
    with SourceGuardian v16.0.0, Copyright (c) 2000-2025, by SourceGuardian Ltd.
    with Zend OPcache v8.1.34, Copyright (c), by Zend Technologies