[FIX] account: don't mistake a tax with a sum of tax repartition factors == 0 for a 0% tax

Consider this example:

- Create a 42% tax, price-included, with two tax repartition lines doing +100 -100
- Create an invoice of 100€ using this tax, and look at the move lines generated

==> Only a base line of 100 and a receivable/payable line of 100 have been created; no tax line.

This is wrong, as we'd expect to see:

- 100 in payable/receivable
- 100 for the base line
- 42 for the +100 tax repartition line
- -42 for the -100 tax repartition line

The two tax lines are missing because the compute_all considers the tax as a 0% one, because of the +100 -100 stuff.

OPW 2716083

closes odoo/odoo#81725

Signed-off-by: Laurent Smet <las@odoo.com>