Add a way to persist domain pathauto disabled state

Problem/Motivation

On a project, we are defaulting domain path pathauto to on, and completely disabling global pathauto. Unfortunately, since the state is either set to TRUE or empty (NULL), there is no way to persist a disabled state.

Steps to reproduce

• Add a form alter to default domain pathauto to enabled using this logic

        $enabled = $this->domainPathautoGenerator->domainPathPathautoGenerationIsEnabled($this->node, $domain_id);

        if ($enabled === FALSE) {
          $this->form['path']['widget'][0]['domain_path'][$domain_id]['pathauto']['#default_value'] = TRUE;
        }

• Now uncheck the pathauto checkbox for a domain and add a custom alias
• Since the state record is now deleted, the above form alter will re-check the pathauto box and wipe out the custom alias

Proposed resolution

Store the 0 state (like pathauto does). It actually looks like this was the original intent given the logic in DomainPathautGenerator::domainPathPathautoGenerationIsEnabled()

    $collection = $this->getDomainPathPathautoStateCollection($entity, $domain_id);
    $value = \Drupal::keyValue($collection)->get($entity->id());
    if($value === NULL) {
      return FALSE;
    }
    return $value;

Remaining tasks

User interface changes

API changes

Data model changes