Allow orders to be completed without any "lines" (#8258)

* Allow orders to be completed without any "lines"

Note: There may be "extra lines" for these orders!

* Fix for unit test

src/backend/InvenTree/order/models.py

@@ -780,7 +780,7 @@ class PurchaseOrder(TotalPriceMixin, Order):
     @property
     def is_complete(self):
         """Return True if all line items have been received."""
-        return self.lines.count() > 0 and self.pending_line_items().count() == 0
+        return self.pending_line_items().count() == 0
 
     @transaction.atomic
     def receive_line_item(
@@ -1076,9 +1076,7 @@ class SalesOrder(TotalPriceMixin, Order):
 
     def is_completed(self):
         """Check if this order is "shipped" (all line items delivered)."""
-        return self.lines.count() > 0 and all(
-            line.is_completed() for line in self.lines.all()
-        )
+        return all(line.is_completed() for line in self.lines.all())
 
     def can_complete(self, raise_error=False, allow_incomplete_lines=False):
         """Test if this SalesOrder can be completed.

src/backend/InvenTree/order/test_api.py

@@ -522,6 +522,11 @@ class PurchaseOrderTest(OrderTest):
 
         self.assignRole('purchase_order.add')
 
+        # Add a line item
+        sp = SupplierPart.objects.filter(supplier=po.supplier).first()
+
+        models.PurchaseOrderLineItem.objects.create(part=sp, order=po, quantity=100)
+
         # Should fail due to incomplete lines
         response = self.post(url, {}, expected_code=400)