[12.0] [BUG] Point of sale doesn't handle 0% VAT taxes

[oyale]

The Point of Sale (POS) module is currently not handling the new VAT taxes in Spain at 0%. This leads to malformed printed tickets and electronic invoices.

Module

Point of Sale / Ticketbai

Describe the bug

The line 1790 at models.js is not properly handling taxes when the amount to be added is 0. This conditional has been implemented to ignore taxes in these cases, which is causing the system to fail when generating invoices through the "Ticket Bai" electronic invoicing system for the Basque Country.

To Reproduce

1. Go to the Point of Sale in an Odoo v12 with products having a 0% VAT tax
2. Complete an order with one product at 0% VAT and any other with a positive VAT tax.
3. Observe that the ticket doesn't show the 0% tax. The XML that's built to electronic invoice management also doesn't include it.

Affected versions:
12.0

Expected behavior
The system should be able to handle taxes with an amount of 0 and generate invoices either printed and through the "Ticket Bai" system showing the 0% VAT applied.

Additional context
The problem was detected using the l10n_es_ticketbai_pos module.