Is it possible to use condition keywords like NULL, IS NOT NULL as condition keywords in field & form rules? The scenario:

3 fields --> 

1. Ordernnumber (text) - always visible in a ticket
2. PO send (date)
3. Inventory done (yes/no)

If the Ordernumber field is empty, the PO send and the Inventory done fields should not be visible

If a value (any value) gets typed into the Ordernumber field, the PO send and the  Inventory done fields should become visible.

I manage to hide the 2 fields at form load. I manage to get them visible when i type something into Ordernumber.

It fails when i have a value in Ordernumber when i leave the ticket and revisit it later. 

The form will always hide the 2 fields PO send and Inventory done, even when a value is typed into Ordernumber (Offcourse: if a value exists in Ordernumber, the 2 other fields should remain visible upon opening the form, and not be hidden...)