Allow parent config to override empty values of more specific config

Tested patch #55, works well.

Patch #55 does not seem to work with a headless approach. Metatags will be saved in the computed field. In a normal approach, where Drupal is rendering the output, the patch overwrites empty tags with global defaults:

        // Merge with global defaults.
        if (strpos($route_name, 'canonical') !== FALSE) {
          $metatags->overwriteTags(metatag_filter_empty_tokens($entity_metatags->get('tags'), $entity));
        }

However when saving to the computed field, the route is not canonical and so global defaults are left untouched. When accessing JSON:API, only the computed field gets accessed and no merge can happen at that time.

I confirm that the patch doesn't work with the JSON:API module.

I updated the latest patch and added the fix for the computed field.