Document the use of a NULL total price for empty carts in 2.x

[rszrama]

Commerce 1.x's function commerce_order_calculate_total() will always initialize a total price with $0 (defaulting to the default currency of the site in a case where there are no items on the order to say otherwise). Commerce 2.x uses NULL, which means you cannot blindly pass the total price field's value to Twig and render it with price formatting - that would result in NaN when the template is rendered. We should document this difference in the section:

https://docs.drupalcommerce.org/v2/developer-guide/adapting-from-1x/