src: use ToLocal in SafeGetenv

This commit replaces the IsEmpty call to use ToLocal instead which
allows for the following ToLocalChecked function call to be avoided.

PR-URL: https://github.com/nodejs/node/pull/33695


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>