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: https://github.com/nodejs/node/issues/25483
PR-URL: https://github.com/nodejs/node/pull/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>