assert: stricter ifError

This makes `assert.ifError` stricter by only accepting `null` and
`undefined` from now on. Before any truthy value was accepted.

PR-URL: https://github.com/nodejs/node/pull/18247


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>