src: fix UB in InternalModuleReadFile()

`&vec[0]` is undefined behavior when `vec.size() == 0`.

It is mostly academic because package.json files are not usually empty
and because with most STL implementations it decays to something that
is legal C++ as long as the result is not dereferenced, but better safe
than sorry.

Note that the tests don't actually fail because of that, I added them
as sanity checks.

PR-URL: https://github.com/nodejs/node/pull/16871


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>