Force empty choices to appear as ghosts

<?php

add_filter( 'facetwp_facet_render_args', function( $args ) {
    if ( 'program_days' == $args['facet']['name'] ) {
        $lookup = [];
        $temp_values = [];
        $days = [ 'monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday' ];

        foreach ( $args['values'] as $row ) {
            $lookup[ $row['facet_value'] ] = $row;
        }

        foreach ( $days as $day ) {
            if ( isset( $lookup[ $day ] ) ) {
                $temp_values[] = $lookup[ $day ];
            }
            else {
                $temp_values[] = [
                    'facet_value' => $day,
                    'facet_display_value' => ucfirst( $day ),
              		'term_id' => 0,
         			'parent_id' => 0,
          			'depth' => 0,
          			'counter' => 0
                ];
            }
        }

        $args['values'] = $temp_values;
    }

    return $args;
});

// Or, to add different values for 'facet_value' and 'facet_display_value':

add_filter( 'facetwp_facet_render_args', function( $args ) {

  if ( 'business_country' == $args['facet']['name'] ) {

    $lookup = [];
    $temp_values = [];
    $items = [
      'england' => 'England',
      'scotland' => 'Scotland',
      'wales' => 'Wales',
      'northern-ireland' => 'Northern Ireland',
      'ireland' => 'Ireland'
    ];

    foreach ( $args['values'] as $row ) {
      $lookup[ $row['facet_value'] ] = $row;
    }

    foreach ( $items as $key => $value ) {
      if ( isset( $lookup[ $key ] ) ) {
        $temp_values[] = $lookup[ $key ];
      }
      else {
        $temp_values[] = [
          'facet_value' => $key,
          'facet_display_value' => $value,
          'term_id' => 0,
          'parent_id' => 0,
          'depth' => 0,
          'counter' => 0
        ];
      }
    }

    $args['values'] = $temp_values;
  }

  return $args;
});