Fix balance calculation

[kochie]

This PR fixes an issue I had where editing an invoice for a service that uses rate and quantity will fail because there is no total value. This will result in a NaN being used to update the db causing a 500 failure on the server.

I'm not sure if this is happening to anyone else. But the reproduction steps for me.

1. Create an invoice for a service with a product with a rate.
2. Try to edit the invoice in any way.
3. Save the invoice.