nixos/tinydns: default data to empty string

(not strictly required to start the service)

(cherry picked from commit 735b41c34f)

nixos/modules/services/networking/tinydns.nix

@@ -15,6 +15,7 @@ with lib;
 
       data = mkOption {
         type = types.lines;
+        default = "";
         description = "The DNS data to serve, in the format described by tinydns-data(8)";
       };