Unpartitioned FS Datasets will create empty files when getWriter is called

When a Dataset is not partitioned, a file writer is returned by the DatasetView#newWriter method. If this writer is opened and closed without writing any data, it will still write a file with just a file header. The FS implementation shouldn't create files unless records are written, even though open is called, open signals that the caller is ready to write, not that they are guaranteed to.