Allow awaiting nullable tasks

[MrTA2]

Consider this code snipper:

Func<Task>? del = GetDelegate();
await del?.Invoke();

IAsyncDisposable? obj = GetDisposable();
await obj?.DisposeAsync();

First version generates a warning and an NRE at runtime, and the second one generates an error, because the method call expression, not having been executed, returns Task? null.

Proposed solution: await adds a check for Task? and ValueTask? - await if not null, do nothing if null.

[HaloFour]

Check out: #35

[MrTA2]

Thanks!