fix: workaround for possible V8 bug fixes #4266

[skilledDeveloper]

workaround for possible V8 bug (due to the use of .ToHandleChecked() in deps/v8/src/api.cc)
Original issue: #4266
@bnoordhuis please review

[bnoordhuis]

A couple of comments:

1. This PR should be against the v0.12-staging branch.
2. Don't use magic constants, add a static const size_t kMaxLength = ... to e.g. ExternString.
3. There is no point in creating the ExternString if you're not going to use it.
4. Returning an empty string is the wrong thing to do. Returning an empty handle may be okay.
5. Comment style: // Capitalized and punctuated.

[bnoordhuis]

Forgot to mention: please add regression tests.

[skilledDeveloper]

OK. I'll create a new PR for this.

[skilledDeveloper]

@bnoordhuis
Does a test like this work?

[bnoordhuis]

Yes, you're welcome to steal tests from the master branch. They'll probably need some adjusting though.