Can I set the default quantity to zero for optional items?

Yes, you can set the default quantity to zero for optional items by adding the following code snippet to your theme functions:

if ( !function_exists( 'yith_wcpb_customization_allow_zero_as_initial_quantity' ) ) {
    add_action( 'yith_wcpb_bundle_items_list_bundle_quantity', 'yith_wcpb_customization_allow_zero_as_initial_quantity', 10, 2 );

    function yith_wcpb_customization_allow_zero_as_initial_quantity( $qty, $min_qty ) {
        return !$min_qty ? 0 : $qty;
    }
}