// This snippet must be more or less public domain, because you can find it from several sources
// It is not coded by me, I just copied the snippet from somewhere
// Paste it in functions.php or use some custom functions plugin like Code Snippet
add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'ec_filter_dropdown_args', 10 );

function ec_filter_dropdown_args( $args ) {
    $args['show_option_none'] = 'Choose...';
    return $args;
}