fix(adguard): rewrite with correct lib.length syntax

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/adguard.nix

@@ -98,18 +98,7 @@ in
           querylog_file_enabled: true
           statistics_enabled: true
 
-        ${lib.optionalString (cfg.clients != []) ''
-          clients:
-            persistent:
-              ${lib.concatStringsSep "\n              " (
-                map (client: ''
-                  - name: ${client.name}
-                    ids:
-                      - ${config.sops.placeholder.${client.idSecret}}
-                '') cfg.clients
-              )}
-        '' + ''
-          + lib.optionalString (lib.length cfg.clients == 0) ''
+        ${lib.optionalString (lib.length cfg.clients == 0) ''
           clients:
             persistent: []