Align the Checkout block's empty-cart fallback with the new Cart and Mini-Cart empty state

The Checkout block renders its own empty-cart state with the old copy and layout, in plugins/woocommerce/client/blocks/assets/js/blocks/checkout/empty-cart/index.js:23: a cart icon, "Your cart is currently empty!", a sentence, and a "Browse store" button. The Cart and Mini-Cart blocks now show "Your cart is empty" with a "Return to shop" button, since woocommerce/woocommerce#68560.

It is rarely seen, because an empty-cart visit to the Checkout page redirects to the Cart page. It shows when the redirect doesn't apply: the Checkout block is on a page other than the configured Checkout page, Cart and Checkout share a page, woocommerce_checkout_redirect_empty_cart returns false, or the cart empties after the page has loaded.

Note: this state renders inside the Page: Checkout template, which uses the minimal Checkout Header (site title and cart link only, no navigation, no footer).

Raised by WOOAIRR-385.