nixos: don't implicitly map missing user groups to `nogroup`

Before: `users.users.user1.group = "group-not-defined-anywhere-else"`
would result in user1 having the primary group `nogroup`, assigned at
activation time and only with a (easy to miss) warning from the
activation script. This behaviour is a security issue becase no files
should be owned by `nogroup` and it allows for unrelated users (and
services) to accidentally have access to files they shouldn't have.

After: The configuration above results in this eval error:
  - The following users have a primary group that is undefined: user1
  Hint: Add this to your NixOS config:
    users.groups.group-not-defined-anywhere-else = {};

0 files changed, 0 insertions, 0 deletions