Improve Empty facet behavior.

Attaching a quick patch which helped me to fix the issue when using the facets_form_checkbox widget.

btw, the code which builds the container with the empty text can be found in Drupal\facets\FacetManager\DefaultFacetManager::build() under the comment

// No results behavior handling. Return a custom text or false depending on
// settings.

Maybe this should be on the widget plugin base e.g. add it's own custom config similar to the Facet's config. And use it instead there.

I've created custom CheckboxesWidget plugin and used such code:

    if (!empty($this->processedItems)) {
      $content = [
        '#type' => 'checkboxes',
        '#options' => $options,
        '#default_value' => $default_value,
        '#after_build' => [[static::class, 'indentCheckboxes']],
        '#depths' => $depths,
        '#ancestors' => $ancestors,
        '#indent_class' => $this->getConfiguration()['indent_class'],
      ];
    }
    else {
      $content = [
        '#type' => 'container',
        '#attributes' => [
          'data-drupal-facet-id' => $facet->id(),
          'class' => ['facet-empty'],
        ],
        'empty_text' => [
          '#markup' => $this->t('No applicable options available.'),
        ],
      ];
    }
    return [
      $facet->id() => [
        '#type' => 'fieldset',
        '#title' => $facet->getName(),
        '#attributes' => [
          'data-drupal-facets-form-ancestors' => Json::encode($ancestors),
        ],
        $facet->id() => $content,
      ],
    ];