account_invoice_import: improve handling of simple PDF invoices

Implement what is described in issue OCA#610:
- account_invoice_import_simple_pdf and account_invoice_import_invoice2data can now be use
d together
- when importing an invoice for an unknown partner, it creates an empty invoice with PDF a
s attachment
- when importing an invoice via mail gateway and the from email is set
on a partner that doesn't have any import config, create a supplier
invoice with PDF as attachment and with that partner
- when importing an invoice via mail gateway and the from email is set
on a partner that has a single-line import config, create a supplier
invoice with PDF as attachment, that partner and 1 invoice line with price_unit=0.

base_business_document_import/models/business_document_import.py

@@ -342,7 +342,7 @@ def _match_partner(  # noqa: C901
             return partner
 
         if not raise_exception:
-            return
+            return None
         raise self.user_error_wrap(
             "_match_partner",
             partner_dict,