[FIX] account: unbalanced entry with price_unit 0

Steps to reproduce:

- Create invoice for a prudict at price X > 0
- Change the price to 0 before saving
- Then save it
-> Error : Cannot create unbalanced journal entry

This bug was introduced in 14.0 with this commit fe7d56dc32c71e04b54de9dbd756a48942a832f4
With this commit, we use an ugly condition to catch the
specific case of a tax with price included and amount type division
to avoid changing the price unit if the line balance is zero.

opw-2822635

closes odoo/odoo#89632

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