At the moment, it doesn't seem possible to set the null/empty. We've tried '' which just adds the quotes to the end, and setting to nothing, which just uses the default .tmp.
We want the in use file to have the same name as the closed file, so we can read from files that are in use without the file moving from underneath us. In our use case, we know that an in use file is still readable and parseable, because it is just text with a JSON document per line.
It looks like the HDFS sink code can handle this change already, but at the moment there is no way to set the bucketPath and targetPath to be the same.