EDIT

  $content_markup = $names_used_in_source_markup . $element_markup . $source_references_markup;

  if(!$content_markup && $feature_block_settings['sources_as_content'] !== 1 || $feature_block_settings['sources_as_content_to_bibliography'] == 1){

    // no textual content, so skip this element completely, even if there could be an footnote key

    // see #4379

    return null;

  }

    $render_array['#value'] = $feature_label . $content_markup;

    $render_array['#value_suffix'] = $annotations_and_sources['foot_note_keys'];

    // sanitize: remove empty and NULL items from the render array

    $tmp_out_array = $elements_out_array;

    $elements_out_array = array();

    foreach($tmp_out_array as $item){

      if(is_array($item) && isset($item[0])){

        $elements_out_array[] = $item;

      }

    }