Use facetwp_template_html to prevent loading the template when no facets are selected

test.php

<?php

function fwp_override_template( $return, $class ) {
    foreach ( $class->ajax_params['facets'] as $facet ) {
        if ( ! empty( $facet['selected_values'] ) ) {
            return false; // don't override
        }
    }
    return ''; // override
}
add_filter( 'facetwp_template_html', 'fwp_override_template', 10, 2 );