PO Charge Account rule should update based on category mapping

Summary:

Our purchase order charge account derives from the requisition charge account. The requisition charge account natural account segment has its own rule derived from item category. We already have added the "null" conditions to the account rule so users can edit the requisition charge account once it is not null. Our users can edit the PO charge account, but if they edit the category on the line, the natural account segment doesn't route back to the category mapping - it copies the requisition charge account. So when a new category is selected on PO, the account segment is not updated (even