soapUI - how to pass null value and empty string in xml

I have just started using soapUI. Could someone please let me know how to pass a empty string and null value in an xml tag?

Here's what I am trying to test:
I am testing a user info submission. The first name is a required field. I want to test passing a empty string for the first name, and passing a null value for the first name...and see that I get back an error.

Thanks..