Fix empty filter list

Updated list_filters_actions block to return nothing if there are no filters defined.

This is important because in standard_layout.html.twig there is a check for emptiness:

  {# in standard_layout.html.twig #}
  {% block sonata_page_content_nav %}
    {% if _tab_menu is not empty or _actions is not empty or _list_filters_actions is not empty %}
    {# ... #}
  {% endblock %}

Prior to this update list_filters_actions was returning the outer list with "Filter ^" but no options in the sub-menu.