fix(adguard): handle empty clients list

Only render clients section when clients are configured.

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

modules/adguard.nix

@@ -98,15 +98,16 @@ in
           querylog_file_enabled: true
           statistics_enabled: true
 
-        clients:
-          persistent:
-            ${lib.concatStringsSep "\n            " (
-              map (client: ''
-                - name: ${client.name}
-                  ids:
-                    - ${config.sops.placeholder.${client.idSecret}}
-              '') cfg.clients
-            )}
+        ${lib.optionalString (cfg.clients != []) ''
+          clients:
+            persistent:
+              ${lib.concatStringsSep "\n              " (
+                map (client: ''
+                  - name: ${client.name}
+                    ids:
+                      - ${config.sops.placeholder.${client.idSecret}}
+                '') cfg.clients
+              )}
 
         filtering:
           protection_enabled: true