src: fix async hooks crashing when there is no node context

This commits basically skips any promise env hooks when there's no node context currently active, which would otherwise end up in a crash.

Fixes: https://github.com/nodejs/node/issues/19104

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

src (async_hooks)