The "Purchased In This Category" email was not working for variable
products. This was happening because MailPoet calls
WC_Product::get_category_ids() to get the list of WooCommerce
categories associated with a given product, but this method doesn't work
for product variations (see
https://github.com/woocommerce/woocommerce/issues/12942).
In this commit, I'm implementing a workaround. I changed the code to
check if the product is a product variation and, if so, get the
categories from the parent product instead of the product itself. But I
also plan to raise this issue with the WooCommerce team as, in my opinion,
it is odd that WC_Product::get_category_ids() doesn't work with product
variations and fails silently.
[MAILPOET-3416]
