facetwp index a value indicating a field is set instead of the field

<?php
/** instead of indexing the value of a field, index a value and label to indicate the custom field exists for that post **/

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'my_facet' == $params['facet_name'] ) { // change my_facet to the name of your facet
        if (!empty( $params['facet_display_value'] ) ) {
            $params['facet_value'] = 'true'; // this will be the value in the filter url
            $params['facet_display_value'] = 'Has Value'; // this is the label you want the facet to show
        } else {
          $params['facet_value'] = ''; // don't index is value is blank
        }
    }
    return $params;
}, 10, 2 );