nixos/chrony: don't emit initstepslew when servers is empty

Setting the server list to be empty is useful e.g. for hardware-only
or virtualized reference clocks that are passed through to the system
directly. In this case, initstepslew has no effect, so don't emit it.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

nixos/modules/services/networking/chrony.nix

@@ -12,7 +12,7 @@ let
     ${concatMapStringsSep "\n" (server: "server " + server) cfg.servers}
 
     ${optionalString
-      cfg.initstepslew.enabled
+      (cfg.initstepslew.enabled && (cfg.servers != []))
       "initstepslew ${toString cfg.initstepslew.threshold} ${concatStringsSep " " cfg.initstepslew.servers}"
     }