nixos/users-groups: fix confusing error message

If we include users with unset groups, we get this very confusing
message, with invalid Nix code:

       - The following users have a primary group that is undefined: qyliss
       Hint: Add this to your NixOS configuration:
         users.groups. = {};

We don't need to include such users in this check, since they'll be
caught anyway by this one:

       - users.users.qyliss.group is unset. This used to default to
       nogroup, but this is unsafe. For example you can create a group
       for this user with:
       users.users.qyliss.group = "qyliss";
       users.groups.qyliss = {};

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/config/users-groups.nix b/nixos/modules/config/users-groups.nix
index 39aac9fb821bd..2aed620eb154c 100644
--- a/nixos/modules/config/users-groups.nix
+++ b/nixos/modules/config/users-groups.nix
@@ -475,7 +475,7 @@ let
   sdInitrdUidsAreUnique = idsAreUnique (filterAttrs (n: u: u.uid != null) config.boot.initrd.systemd.users) "uid";
   sdInitrdGidsAreUnique = idsAreUnique (filterAttrs (n: g: g.gid != null) config.boot.initrd.systemd.groups) "gid";
   groupNames = lib.mapAttrsToList (n: g: g.name) cfg.groups;
-  usersWithoutExistingGroup = lib.filterAttrs (n: u: !lib.elem u.group groupNames) cfg.users;
+  usersWithoutExistingGroup = lib.filterAttrs (n: u: u.group != "" && !lib.elem u.group groupNames) cfg.users;
 
   spec = pkgs.writeText "users-groups.json" (builtins.toJSON {
     inherit (cfg) mutableUsers;