getPurchasedEntity can return NULL and there can be error

@khiminrm thanks, please turn this into a MR.

The actual problem is calling functions on $entity when it is null. Switching to using $order_item->getPurchasedEntityId() and $order_item->getTitle() handles the lack of a purchased entity gracefully and also works on non-product purchased entities, returning the name and entity id. getPurchasedEntityId() returns null instead of throwing an error if there is no purchased entity.

I also moved the SKU code so it uses a null coalescing operator to determine if it should use the sku or the purchased entity id instead of overwriting values previously set.

I made a small change so it returns an empty string if the id is null

It's a stacked null coalescing operator so if $sku is null then it uses $order_item->getPurchasedEntityId() and if that's also null it uses an empty string.

@rhovland looking great!

Just one last step to finish this: Is an empty string an accepted value for Facebook?
'content_name' => $order_item->getTitle() will then still return the title, right? (Hopefully we can be sure there, it's not NULL :P)

According to the order item entity interface it will always return a string. Titles are required for order items in the UI, you cannot save one without it.

  /**
   * Gets the order item title.
   *
   * @return string
   *   The order item title
   */
  public function getTitle();

Thanks @rhovland, so let's merge this!

Automatically closed - issue fixed for 2 weeks with no activity.