Show zero lines in an empty sale

[sridharkalaibala]

The empty-sale placeholder was counted as an item line after loading the till. Count only rows with an item identity. Desktop payment integrity tests pass (33 tests); browser verification covers empty and populated carts.

[github-actions]

Merged to develop. Anyone can test this - you do not need write access.

Try it at https://develop.posnic.io, or run it yourself:

git fetch origin develop && git checkout develop
npm install && npm --prefix api install
npm run dev   # then http://localhost:3000

When you have tested it, say what you did and what happened, and set
QA passed or QA failed. If you cannot set labels, just comment -
a maintainer will.

Reporting that something is broken is as useful as fixing it. It is
better found here than by a shopkeeper.