Index and filter if a custom field value is empty or not

<?php
// See: 
// https://facetwp.com/help-center/developers/hooks/indexing-hooks/facetwp_index_row/#filter-posts-with-a-non-empty-custom-field

add_filter( 'facetwp_index_row', function( $params, $class ) {
  if ( 'my_facet_name' == $params['facet_name'] ) { // Replace "my_facet_name" with the name of your facet
    // When custom field is not empty
    if ( $params['facet_value'] !== '' ) {
      $params['facet_value'] = 'yes';
      $params['facet_display_value'] = 'Yes - has a value';
    }
    // When custom field is empty
    else {
      $params['facet_value'] = 'no';
      $params['facet_display_value'] = 'No - does not have a value';
    }
  }
  return $params;
}, 10, 2 );