distro detection breaks apkg on prerelease distros

When the local distro version cannot be detected (e.g. Debian testing/sid/...) distro.version() returns 'n/a', which creates no end of trouble for apkg down the line (/ is a directory separator, and these versions don't parse in DistroRule). I would suggest a change in behaviour where:

• this generates a "missing" version (distro.version is None)
• a missing version actually sorts after any other version (and this is the controversial part:) so a distro.match('debian > 10; != 12') would match on sid (while distro.match('debian <= 10') wouldn't, no change from what we do now)

Sorry for spamming your CI with test pushes in the MUT MR but most of the tests fail for me locally because of this (my dev environment is a testing/sid/experimental mix).