Display empty cart if not displaying non-current carts

Problem/Motivation

The empty cart message ("Your shopping cart is empty") is not displayed when there are no current carts and display_non_current_carts is set to fault, i.e. the page is blank.

Steps to reproduce

1. Create a cart
2. Set is as non-current
3. Ensure there is nothing in the current cart
4. Untick "Show non-current carts" in Cart Advanced Settings
5. View cart page

Proposed resolution

Check whether non-current carts are set to be displayed when deciding whether to display the empty cart message.

Remaining tasks

User interface changes

API changes

Data model changes