src: use Isolate::TryGetCurrent where appropriate

In two places, we call `Isolate::GetCurrent()` even though that is
technically invalid usage of the function.
Now that V8 exposes `Isolate::TryGetCurrent()`, we can do this
in a proper way.

PR-URL: https://github.com/nodejs/node/pull/39954


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>