job-order-editor: Check if purchase order number is not null

Removes a warning when the job order has no purchase order.
job-order-editor: Update purchase order number in a private method

This will lessen the code and makes sure that it is properly updated
when a new purchase order is selected.
database: Retrieve purchase order and invoice reference number

This is necessary to tell if the purchase order and invoice is already
released.
job-order-list-view: Show "Unreleased" if purchase order is not yet released