WooCommerce Product Vendors 2.3.2

* Fix - Added validation for default product cost and handling fee fields.
* Fix - Show refunded quantity in the vendor order details.
* Fix - Show shipping method name in the order email and dashboard if shipping is passed to vendor.
* Tweak - WC 5.8 compatibility.
* Tweak - WP 5.8 compatibility.

* Fix - Show refunded quantity in the vendor order details.
* Fix - Show shipping method name in the order email and dashboard if shipping is passed to vendor.

* Add - New action on Vendor Profile screen, `wcpv_vendor_settings_render_additional_fields`, to enable other plugins to add additional settings fields.
* Fix - Ensure array key exists before we try using it.
* Fix - Ensure the data we are using is an array before we try accessing a property of that array.