Issue: With a custom tax computation that depends on the quantity, when
the price of the item is 0, when changing the quantity,
the taxes are not changed

Steps to reproduce :
 1) Install Accounting, and account_tax_python
 2) Accounting > Configuration > Accounting > Taxes : create
 a task :
   Tax Computation: Python Code
   Python code : `result = quantity * 0.3`
 3) Accounting > Customer Invoices > New Invoice
 4) Add a line with a product, unit price = 0, quantity = 2
 5) Change the quantity, the tax does not change

Why is that a bug:
 When the price is 0, changing the quantity doesn't affect
 the price so the onchange is not triggered even though the
 line changed

opw-2559200

closes odoo/odoo#75086

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>