http2: optimize neverIndex SafeSet creation

Only create SafeSet when sensitiveHeaders array is non-empty.
This optimizes the common case where sensitiveHeaders is empty
by avoiding Set creation overhead in the hot path.

The optimization maintains correctness for all cases while
providing significant performance improvement for the common
case where sensitiveHeaders is not explicitly configured.

Author: Ayoub-Mabrouk

lib/internal/http2/util.js

@@ -761,7 +761,13 @@ function buildNgHeaderString(arrayOrMap,
 
   const singles = new SafeSet();
   const sensitiveHeaders = arrayOrMap[kSensitiveHeaders] || emptyArray;
-  const neverIndex = sensitiveHeaders.map((v) => v.toLowerCase());
+  let neverIndex = null;
+  if (sensitiveHeaders.length > 0) {
+    neverIndex = new SafeSet();
+    for (let i = 0; i < sensitiveHeaders.length; i++) {
+      neverIndex.add(sensitiveHeaders[i].toLowerCase());
+    }
+  }
 
   function processHeader(key, value) {
     key = key.toLowerCase();
@@ -787,7 +793,7 @@ function buildNgHeaderString(arrayOrMap,
         throw new ERR_HTTP2_HEADER_SINGLE_VALUE(key);
       singles.add(key);
     }
-    const flags = neverIndex.includes(key) ?
+    const flags = (neverIndex !== null && neverIndex.has(key)) ?
       kNeverIndexFlag :
       kNoHeaderFlags;
     if (key[0] === ':') {