nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/config/pulseaudio.nix

@@ -306,6 +306,7 @@ in {
         description = "PulseAudio system service user";
         home = stateDir;
         createHome = true;
+        isSystemUser = true;
       };
 
       users.groups.pulse.gid = gid;