os: add fallback for undefined CPUs

For an unsupported OS, a call to os.cpus() throws an error
within os.cpus() itself where it tries to get the length of it.
This fixes the issue by adding fallback for undefined CPUs.

Fixes: nodejs#25483
PR-URL: nodejs#25493
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/os.js

@@ -89,7 +89,8 @@ function loadavg() {
 }
 
 function cpus() {
-  const data = getCPUs();
+  // [] is a bugfix for a regression introduced in 51cea61
+  const data = getCPUs() || [];
   const result = [];
   for (var i = 0; i < data.length; i += 7) {
     result.push({