<?php

// Add to your (child) theme's functions.php
add_action( 'wp_footer', function() {
?>
<script>
(function($) {
  $(document).on('facetwp-loaded', function() {
    $('.facetwp-facet-resource_topics').prepend('<div class="facetwp-radio" data-value="">All</div>');
  });
})(jQuery);
</script>
<?php
}, 100 );