Use boolean value when enforcing value to edit delivery date of purchase line

The clients always convert Boolean field values into boolean so the domain
validation against None value is always failing.

Closes #12306

modules/purchase/purchase.py

@@ -1216,7 +1216,7 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
         "Edit Delivery Date",
         domain=[
             If(Eval('type') != 'line',
-                ('delivery_date_edit', '=', None),
+                ('delivery_date_edit', '=', False),
                 ()),
             ],
         states={