getCartItems() returns a Collection and an empty Collection is truthy, so an
empty cart became an order with only shipping and payment costs. Adds an
end-to-end checkout submit test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJxEe79eJY48d3EUEZRwuC