asyncMapSample crashes on null values

[Leon Senft]

I tried using asyncMapSample on a Stream<void> (to which I'm passing null since the data doesn't matter) and found that it crashes immediately due to the following null assertion:

stream_transform/lib/src/aggregate_sample.dart

Line 31 in fe80477

 controller.add(currentResults!); 

One can work around this issue for a Stream<void> by passing a non-null object since the value doesn't matter, but I suspect this prevents this extension method from being used on a stream of nullable values (e.g. Stream<String?>).