Fix empty-object false negative in links page no-data check

The check `!data.hosts && !data.unifi_switches` never caught empty
objects `{}`, which are truthy. Replace with Object.keys length checks
so the friendly "no data yet" banner renders when both collections
are empty.

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

templates/links.html

@@ -548,7 +548,7 @@ function checkLinksStale(updatedStr) {
 async function loadLinks() {
   try {
     const data = await lt.api.get('/api/links');
-    if (!data.hosts && !data.unifi_switches) {
+    if ((!data.hosts || !Object.keys(data.hosts).length) && (!data.unifi_switches || !Object.keys(data.unifi_switches).length)) {
       document.getElementById('links-container').innerHTML =
         '<div class="link-no-data">No link data yet — monitor has not completed a full cycle.</div>';
       return;