Empty 'replace' patterns for MachineName render element causes PHP Warning: preg_match(): Compilation failed: nothing to repeat at offset 1

Problem/Motivation

When an empty string is used in the replace key of the core machine_name form element, it causes a PHP warning to appear in logs:

Warning: preg_match(): Compilation failed: nothing to repeat at offset 1 in /var/www/html/docroot/core/lib/Drupal/Core/Render/Element/MachineName.php on line 240

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...

Notably it still works, but the warning is due to an assumption that a character will be set.

Steps to reproduce

Create a custom form with a machine_name element that is configured to replace invalid characters with an empty string. For example, this element creates a subdomain from another site_name textfield element. Domains cannot include underscores (the default machine name replace character), so this element serves to remove invalid characters.

    $form['subdomain'] = [
      '#type' => 'machine_name',
      '#title' => $this->t('Subdomain'),
      '#machine_name' => [
        'exists' => [$this, 'subdomainExists'],
        'label' => $this->t('Subdomain'),
        'source' => ['site_name'],
        'replace_pattern' => '[^a-z0-9-]+',
        'replace' => '',
        'error' => $this->t('The subdomain can only contain letters, numbers, and dashes.'),
      ],
    ];
<code>

<h3 id="summary-proposed-resolution">Proposed resolution</h3>

Add an <code>empty()

check before the preg_match() (see attached patch).

Remaining tasks

1. Open a merge request with a fix.
2. Add tests.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

TBD