src: do not crash when accessing empty WeakRefs

Making `.incRef()` and `.decRef()` fail silently leads to better error
messages when trying to access the underlying value (as opposed to
crashing inside these methods).

Refs: https://github.com/nodejs/node/pull/25461#issuecomment-524481482

PR-URL: https://github.com/nodejs/node/pull/29289


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gus Caplan <me@gus.host>