src: handle empty Maybe in uv binding initialize

This can fail when terminating a Worker that loads
the `uv` binding at the same time.

Refs: https://github.com/nodejs/node/pull/25061#issuecomment-447648475
Fixes: https://github.com/nodejs/node/issues/25134
PR-URL: https://github.com/nodejs/node/pull/25079


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>