fs: only use Buffer.concat in promises.readFile when necessary

PR-URL: #37127
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

lib/internal/fs/promises.js

@@ -311,7 +311,7 @@ async function readFileHandle(filehandle, options) {
       chunks.push(buffer.slice(0, bytesRead));
   } while (!endOfFile);
 
-  const result = Buffer.concat(chunks);
+  const result = chunks.length === 1 ? chunks[0] : Buffer.concat(chunks);
 
   return options.encoding ? result.toString(options.encoding) : result;
 }