Fix the Debian packaging script on systems with no codename

When calling `make deb-pkg` on a system with no codename (for example
Arch Linux), lsb_release sometimes outputs `n/a` as the codename.

This breaks dpkg-parsechangelog, which can't process the changelog
correctly.

Signed-off-by: Marcin Mielniczuk <marmistrz.dev@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>