With async_hooks it's also possible that the "current microtask" is an
await task, whose generator is already suspended, when there's an
exception thrown in the AFTER callback. In that case we cannot build
a meaningful async stack trace.

Bug: chromium:897406, v8:7522
Change-Id: I682dc1fc3ebb1864e1c2061041ff99ced0313f0c
Reviewed-on: https://chromium-review.googlesource.com/c/1292057Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56839}