[FIX] l10n_latam_invoice_document: Allow digitalization

Steps to reproduce:

  - Install l10n_ar module (or any l10n_* module that depends on
    l10n_latam_invoice_document)
  - Go to Accounting -> Vendors -> Bills
  - Click on Upload and select a bill file
  - Open draft bill
  - Click on `Send For Digitalization`

Issue:

  Digitalization don't start and a warning message is diplayed saying
  that some required fields are invalid.

Cause:

  When clicking on the button to digitalize the invoice, we try
  to save the form first before executing the action.
  By doing so, if some fields are required and empty, a warning message
  will be displayed.

  In this case, the issue is with the fields `Document Type` and
  `Document Number`.

Solution:

  Set `Document Type` and `Document Number` to not required if there is
  no partner_id set on the bill.

opw-2797444

closes odoo/odoo#96004

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