Fix empty Packages file for APT repository.
Fix empty Packages file for APT repository.

This change also adds an extra sanity check to the make_apt.sh script,
in order to ensure that this simple mistake does not occur again.

PiperOrigin-RevId: 355101754

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor