readline: error on falsy values for callback

It was intended, according to in-test comments and common behaviour,
that callbacks be either `undefined` or a function, but falsy values
were being accepted as meaning "no callback".

PR-URL: https://github.com/nodejs/node/pull/28109


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>