[FIX] l10n_it_delivery_note: fix line display_type in invoice generation

Author: Borruso

l10n_it_delivery_note/models/stock_delivery_note_line.py

@@ -217,7 +217,7 @@ def _prepare_invoice_line(self, **optional_values):
             price_unit = self.price_unit
 
         res = {
-            "display_type": "product",
+            "display_type": self.display_type or "product",
             "name": product_name,
             "product_id": self.product_id.id,
             "product_uom_id": self.product_uom_id.id,