In my opinion, the following behavior is wrong.
Product: soapUI 3.6
Platform: Windows XP
Test step type: HTTP
It's not possible to send an empty string value when using the QueryString in a HTTP request. When a parameter in the QueryString has the empty string value, the parameter itself is ignored (not sent), which at the end is received as null at the web server.
Works fine if you don't use the QueryString and send the parameters in the HTTP body. For example