child_process: harden fork arguments validation

Ensure that the first argument `modulePath` of `fork` method
must be provided and be of type string.

PR-URL: https://github.com/nodejs/node/pull/27039


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>