Reading parquet files using dataset scanner may stall due to a never-finished future.
To reproduce this case, one needs two parquet files and sets the filter expression to something that could filter one file completely. After that, calling `AsyncScanner::ToRecordBatchReader` and read data continually.
I also have dug this bug a little. It's caused by the `MakeEmptyGenerator<std::shared_ptr<RecordBatch>>` when filtered row groups is empty, which's ignored by `FragmentToBatches` and causes SequencingGenerator to stall.
A quick fix is to return a record batch with 0 rows instead of returning a nullptr there.