nixos/nsd: Improve checking for empty dnssec zones

While at it (see previous commit), using attrNames in combination with
length is a bit verbose for checking whether the filtered attribute set
is empty, so let's just compare it against an empty attribute set.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/modules/services/networking/nsd.nix

@@ -435,7 +435,7 @@ let
 
   dnssecZones = (filterAttrs (n: v: if v ? dnssec then v.dnssec else false) zoneConfigs);
 
-  dnssec = length (attrNames dnssecZones) != 0; 
+  dnssec = dnssecZones != {};
 
   dnssecTools = pkgs.bind.override { enablePython = true; };