src: avoid unnecessary ToLocalChecked calls

This commit removes two unnecessary ToLocalChecked calls in
StringBytes::Encode.

PR-URL: https://github.com/nodejs/node/pull/33824


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>