Don't crash if segment cannot be resolved in `captureAsyncFunc`

In the case `captureAsyncFunc` is given an invalid segment (which can be supported with some error logs in other capture solutions) the execution function is called without any support for `subsegment.close()`.
I might make sense to pass a dummy object with a close function to not crash the client code, and thus avoiding to systematically call `subsegment && subsegment.close()`

Start by locating the captureAsyncFunc entry point and trace how it handles an invalid segment before calling the execution function. Confirm the behavior with the repository's existing test setup. Done means client code can safely call subsegment.close() without crashing when the segment cannot be resolved.

Written by the indexing model from the issue text.