[FIX] l10n_it_edi: Generate correct XML for negative invoice lines

When creating an invoice with a positive line and a negative line, with
different taxes, the DatiRiepilogo node for the tax of the negative
line contained positive amounts when they should be negative.

This is because we were applying `abs()` too naively in the XML template
and in the code of _l10n_it_edi_prepare_fatturapa_tax_details.

This bugfix commit changes the logic to no longer use abs().

opw-3316300

closes odoo/odoo#121933

Signed-off-by: Quentin De Paoli <qdp@odoo.com>