src: use MaybeLocal::FromMaybe to return exception

This commit suggests using MaybeLocal::FromMaybe to return the exception
string, passing in a default value (an empty Local<Value>) which will be
used if the exception object is empty.

PR-URL: https://github.com/nodejs/node/pull/33514


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>