Fix(ui): Enter on an empty MAC Lookup input does nothing

The shared parser rejects an empty string, so Enter in the empty field
showed "Invalid physical address". onSubmit now returns early on empty
input, as IP Calculator does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/components/advanced-tools/MacChecker.vue

@@ -178,10 +178,11 @@ const validateInput = (input) => {
 };
 
 const onSubmit = () => {
+    const raw = queryMAC.value.trim();
+    if (!raw) return;
     trackEvent('Section', 'StartClick', 'MACChecker');
     errorMsg.value = '';
     macCheckResult.value = {};
-    const raw = queryMAC.value.trim();
     picked.value = EXAMPLES.some((e) => e.input === raw) ? raw : '';
     const query = validateInput(raw);
     if (query) getMacInfo(query);