nix gc service: fix use of startAt

`startAt = ""` as in `startAt = optionalString false ...` results
in an invalid timer unit (due to "" being promoted to a singleton
list and not filtered out).

Ref: c9941c4b5e

nixos/modules/services/misc/nix-gc.nix

@@ -53,7 +53,7 @@ in
     systemd.services.nix-gc =
       { description = "Nix Garbage Collector";
         script = "exec ${config.nix.package.out}/bin/nix-collect-garbage ${cfg.options}";
-        startAt = optionalString cfg.automatic cfg.dates;
+        startAt = optional cfg.automatic cfg.dates;
       };
 
   };