WooCommerce PDF Vouchers - WordPress Plugin 4.9.9

* [+] New: Optimized the plugin code for not saving the meta if it's blank and also removed unnecessary meta from the DB.

* [*] Fix: Fixed issue of Custom CSS not applying on the PDF voucher templates.
* [*] Fix: Fixed the issue of voucher coupon discount shows greater than cart total.
* [*] Fix: Fixed an issue of multiple time redemption when order amount is less then coupon value.

* [*] Fix: Resolved an issue of gift email notifications not working with the latest WooCommerce 4.9.0.