10 years in an attic.. if your attic is like mine, its cold in winter and hot in summer, its hard on data.
I would of expected getting corrupted video files, but you seems to say the dvr cam and pc driver both agree the disk are empty.
if there was any data written to a DVD-R disk you cannot reformat them, (only DVD-RW can do that).
Which leads me to believe one of two things happened;
1) as My parents, they did not understand how to record, and never checked after recording their first video,
thus kept blanks while making cases.
2) someone in your family decided to swap all recorded DVD-R disks with blank ones leaving the cases, removing some secret no one will never see
I vote #1, my parents have done this, as well as my father in law.