Update intrastat_product_declaration.py

@cubells
I changed the check on 'price_unit' with value into price_subtotal.
By doing so we also cover the case of invoice lines with 100% discount.
Let me know if it's ok so that we can merge (and forward port to v10)

intrastat_product/models/intrastat_product_declaration.py

@@ -501,7 +501,7 @@ class IntrastatProductDeclaration(models.Model):
             total_inv_weight = 0.0
             for inv_line in invoice.invoice_line:
 
-                if not inv_line.price_unit:
+                if not inv_line.price_subtotal:
                     continue
                 
                 if (