[WooCommerce] Make the VAT field required for select countries

wc-eu-vat-select-countries.php

add_action('woocommerce_checkout_process', 'wc_force_vat_field_to_be_not_empty');

function wc_force_vat_field_to_be_not_empty() {
    global $woocommerce;

    // Define countries where VAT field is mandatory
    $eu_countries = array('AT', 'BE', 'BG', 'CY', 'CZ', 'DE', 'DK', 'EE', 'ES', 'FI', 'FR', 'GB', 'GR', 'HU', 'IE', 'IT', 'LT', 'LU', 'LV', 'MT', 'NL', 'PL', 'PT', 'RO', 'SI', 'SK'); 

    // Check if customer is in one of those countries
    if( in_array( $woocommerce->customer->get_country(), $eu_countries ) ) {
        // Check if VAT field isn't empty
        if ( ! $_POST['vat_number'] )
            // Create notice
            wc_add_notice( __( 'Please enter your VAT number.' ), 'error' );
    }
}