Fix tools page: return empty string instead of nil/404

The tools page function returned nil when no child route matched,
causing a 404. Now returns empty string so the layout renders.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -608,7 +608,7 @@
         :title (unquote (or title "SX Tools"))
         (splice-unquote args)))))
 
-(define tools (fn (content) (if (nil? content) nil content)))
+(define tools (fn (content) (or content "")))
 
 (define
   services