Do not allow to save a file that doesn't contain any pages
Description
It is possible to "save document to a file" even if there are no scanned pages. The pdf will be created and evince shows a message that the document does not contain any pages.
We should not allow the user to save an empty file. Maybe we could just make the "save" button insensitive?
Version
Flatpak artifact from commit 7d5bde9b from master