While a zero value invoice has no relevance from a purely accounting viewpoint the lack does mean, for example, that you can’t automatically create a packing list.
There have been a number of posts about this over the years but the feature request hasn’t had much support. You can add your vote at Zero cost invoices
Incidentally you can have zero value lines on an invoice as long as the first line has a value and the total value exceeds zero. (eg buy A and get B free)