[IMP] l10n_in_edi: negative discount now allowed

Before this commit:
Negative discounts could be applied to products, which is not allowed
in Indian EDI.

After this commit:
Applied conditions to check for negative discounts and raise an error in case of
such discounts.

task_id: 3293247

closes odoo/odoo#120218

Signed-off-by: Josse Colpaert <jco@odoo.com>