Enable EU VAT field when the current page contains a OPC form and the cart is empty

enable_eu_vat_field_opc.php

<?php

// Make sure that EU VAT Field is displayed on OPC pages (when the cart is empty)
function enable_eu_vat_field_opc_defualt( $default, $option, $passed_default ) {
	if ( is_wcopc_checkout() ) {
		return 'yes';
	}
	return $default;
}
add_filter( 'default_option_woocommerce_eu_vat_number_b2b', 'enable_eu_vat_field_opc_defualt', 11, 3 );

// Make sure that EU VAT Field is displayed on OPC pages (when the cart is empty)
function enable_eu_vat_field_opc_option( $value, $option ) {
	if ( is_wcopc_checkout() ) {
		return 'yes';
	}
	return $value;
}
add_filter( 'option_woocommerce_eu_vat_number_b2b', 'enable_eu_vat_field_opc_option', 11, 2 );