fix(adguard): fix template string concatenation

Properly concatenate optionalString with content.

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

modules/adguard.nix

@@ -97,8 +97,8 @@ in
           querylog_enabled: true
           querylog_file_enabled: true
           statistics_enabled: true
-
-        ${lib.optionalString (cfg.clients != []) ''
+        ''
+        + lib.optionalString (cfg.clients != []) ''
           clients:
             persistent:
               ${lib.concatStringsSep "\n              " (
@@ -108,6 +108,8 @@ in
                       - ${config.sops.placeholder.${client.idSecret}}
                 '') cfg.clients
               )}
+        ''
+        + ''
 
         filtering:
           protection_enabled: true