When adding an invoice, you can select the check box to Include projects without unbilled data and then select the project on the subsequent screen List tab. This will give you a blank invoice.
Here is the step where you generate the $0 invoice:
The UNBILLED column is null, which means there is no posted data that will show up on the invoice. As a result, you have a $0 invoice to which you may add additional items.
Select the check box for the appropriate project from the list and click Generate Invoices, then click List on the next screen.
Here is the list showing the newly created $0 invoice: