assert: use a default message in assert

In case no arguments are passed to `assert.ok` it should just
use a default message. Otherwise `assert.ok` can not be used as
a callback.

PR-URL: https://github.com/nodejs/node/pull/18319


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>