Some users were getting the PHP notice below when using the
user-switching (https://wordpress.org/plugins/user-switching/) plugin
to switch to a different user.
```
PHP Notice: get_cart was called incorrectly. Get cart should not be called before the wp_loaded action.
```
This notice is generated by WooCommerce when `WC_Cart::get_cart()` is
called before the `wp_loaded` action. user-switching calls a WooCommerce
method to empty the user session and the cart when switching users and
this happens before wp_loaded. MailPoet hooks into the
woocommerce_cart_emptied action
(c9ca134d30/lib/AutomaticEmails/WooCommerce/Events/AbandonedCart.php (L118))
to decide whether it should schedule or cancel an abandoned cart email.
When doing so, it calls WC_Cart::is_empty() which calls
WC_Cart::get_cart(). This is why the PHP notice is generated.
When the cart is emptied we don't need to check it again calling
WC_Cart::is_empty(). So, to get rid of the PHP notice, this commit adds an
extra condition to the if statement to only run WC_Cart::is_empty() if
the current action is not `woocommerce_cart_emptied`.
[MAILPOET-3374]
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]
