Add “0” to range posts with no value

<?php

// Add to your (child) theme's functions.php
// Replace "my_range" with your actual facet name

add_filter( 'facetwp_indexer_row_data', function( $rows, $params ) {
    if ( 'my_range' == $params['facet']['name'] && empty( $rows ) ) {
        $new_row = $params['defaults'];
        $new_row['facet_value'] = 0;
        $new_row['facet_display_value'] = 0;
        $rows[] = $new_row;
    }
    return $rows;
}, 10, 2 );