benchmark: output JSON-compatible numbers

This is to simplify the implementation of a JavaScript version of the
compare.R script.

PR-URL: #38778
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

benchmark/common.js

@@ -267,7 +267,7 @@ class Benchmark {
 function nanoSecondsToString(bigint) {
   const str = bigint.toString();
   const decimalPointIndex = str.length - 9;
-  if (decimalPointIndex < 0) {
+  if (decimalPointIndex <= 0) {
     return `0.${'0'.repeat(-decimalPointIndex)}${str}`;
   }
   return `${str.slice(0, decimalPointIndex)}.${str.slice(decimalPointIndex)}`;