Concatenate - how to get rid of defult "null null" values in field.

I have successfully concatenated the first and last name fields in form design but when I open/run the form the concatenated field (FullName) defaults to "null null" on the form. I want it to be blank until the user enters data in the FirstName and LastName fields.

I am not an experienced Java scripter and need to have the full script exactly as it needs to be entered from start to finish.

Here is what is in the script editor the current script:

form1.Subform1.FullName::calculate - (JavaScript, client)

var first = form1.Subform1.FirstName.rawValue;

var last = form1.Subform1.LastName.rawValue;

this.rawValue = first + " " + last;

LiveCycle Designer ES2

Windows 8

Message was edited by: PenEliz1 I added a screen shot.