Yes, they are generated by default. 

However, Chargebee can be configured to not generate zero dollar invoices (each line item being $0) using the "Hide Zero Value Line Items" option.  Just navigate to Settings > Billing Rules (under Billing section) > enable "Hide Zero value Line Items"option >  hit Update Billing Rules.

Here's a screenshot.

 If credits and discounts are added to an invoice to bring the total payable amount to $0, the invoice will still be generated.