Add getCompletedOrNull to Deferred<T>

[owochle]

Use case
In addition to the experimental Deferred<T>::getCompleted and Deferred<T>::getCompletionExceptionOrNull, it would be very useful to have getCompletedOrNull as a way to reduce this snippet.

if (deferred.isCompleted) {
    deferred.getCompleted()
} else {
    null
}

This way, it would be trivial to get an awaitable value from synchronous code and cleaner than the above snippet, or catching the IllegalState thrown by getCompleted