Use purchase line unit when invoice line has no unit

Closes #12462
(grafted from 582bbe2add15d9d856b3a9315b8c8434f66b7248)

--HG--
branch : 6.0

modules/purchase/purchase.py

@@ -1615,8 +1615,9 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
             if invoice_line.type != 'line':
                 continue
             if invoice_line not in skips:
-                quantity += Uom.compute_qty(invoice_line.unit,
-                    invoice_line.quantity, self.unit)
+                quantity += Uom.compute_qty(
+                    invoice_line.unit or self.unit, invoice_line.quantity,
+                    self.unit)
         return quantity
 
     def _get_invoice_line_moves(self):
@@ -1714,13 +1715,14 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
             moved_quantity *= -1
         invoiced_quantity = 0
         for invoice_line in self.invoice_lines:
-            if (
-                    (not invoice_line.invoice
-                        or invoice_line.invoice.state != 'cancelled')
-                    and self.unit and invoice_line.unit):
-                invoiced_quantity += Uom.compute_qty(
-                    invoice_line.unit, invoice_line.quantity, self.unit,
-                    round=False)
+            if (not invoice_line.invoice
+                    or invoice_line.invoice.state != 'cancelled'):
+                if self.unit:
+                    invoiced_quantity += Uom.compute_qty(
+                        invoice_line.unit or self.unit, invoice_line.quantity,
+                        self.unit, round=False)
+                else:
+                    invoiced_quantity += invoice_line.quantity
         actual_quantity = max(moved_quantity, invoiced_quantity, key=abs)
         if self.unit:
             actual_quantity = self.unit.round(actual_quantity)