tools: fix eslint isRequired

Currently the call can lead to a TypeError with the message:
`Cannot read property 'value' of undefined`.

This fixes it by first checking that the first argument is truthy.

PR-URL: https://github.com/nodejs/node/pull/18729


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>