With the GAE Bulk Uploader script, how do I deal with empty values in the CSV?

Question

I've set up my app.yaml and data_uploader files as suggested in this document. My CSV file has some null values (the spreadsheet that I exported had some empty cells).

When I run the script, I get this error in the log file: [ERROR ] Error in WorkerThread-0: Value should not be empty; received [].

My guess is that it is because some values are empty in the csv file, How can I make sure that the empty values are either imported as '' into the datastore, or not imported at all (the model properties are all optional).

Thanks, David.