doc: note that randomBytes throws when passed null

Fixes: https://github.com/nodejs/node/issues/16778

PR-URL: https://github.com/nodejs/node/pull/17594
Fixes: https://github.com/nodejs/node/issues/16778


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>