use the empty field consistently throughout the data sets

userstats-combined.csv is the only csv-data-set provided on Metrics web that uses the double quotation mark enclosed empty string ""; all others simply use an empty field. Example row 2017-01-09,bridge,sg,scramblesuit,"",10,0,8 from userstats-combined.csv, and 2011-03-06,relay,af,,,,,170,11 from clients.csv

The empty field should also be used in userstats-combined.csv.