Fieldable panel panes do not always define a title when rendered

See #1781264: Rendering panes that do not have $block->title defined causes a PHP notice.

Fieldable panel panes do not always define $block->title:

    $block = new stdClass();
    if (empty($settings['extra_fields']['display']) || !empty($settings['extra_fields']['display']['title']['default']['visible'])) {
      $block->title = !empty($entity->title) ? filter_xss_admin($entity->title) : '';
    }
    // If the above if fails, the $block->title is never set.