async_hooks: separate missing from default context

When context is missing the executionAsyncId will be zero. For the
default triggerAsyncId the zero value was used to default to the
executionAsyncId. While this was not technically wrong because the
functions are different themself, it poorly separated the two concepts.

PR-URL: https://github.com/nodejs/node/pull/17273


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>