ACF – set “Other data source” equal to “Data source” if empty

<?php

// Add to your (child) theme's functions.php

add_filer( 'facetwp_acf_display_value', function( $value, $params ) {
    if ( ! empty( $params['facet_name'] ) ) {
        $facet = FWP()->helper->get_facet_by_name( $params['facet_name'] );
        if ( ! empty( $facet['source_other'] ) && empty( $value ) ) {
            $value = $params['facet_value'];
        }
    }
    return $value;
}, 20, 2 );