Blank out product SKU when cloning a product

This was effectively being done before for the product's sku (stored on the master variant) via the #duplicate_variant method, but now it needs to be done explicitly on the product in #duplicate_product

(cherry picked from commit 0f253bb2a0)

lib/spree/core/product_duplicator.rb

@@ -21,6 +21,7 @@ module Spree
         product.dup.tap do |new_product|
           new_product.name = "COPY OF #{product.name}"
           new_product.taxons = product.taxons
+          new_product.sku = ""
           new_product.created_at = nil
           new_product.deleted_at = nil
           new_product.updated_at = nil