With SetNullString you can set the default value to be put when no assign is made for a variable.
the default is to wipe it out, but you can set it up to put a custom string.

by default, if some variables weren't assigned a value, they simply disappear from the parsed html:

full name:  
e-mail:  
income:  

ok, now let's assign a nullstring:

full name:  
e-mail:  
income:  

custom nullstring for a specific variable and default nullstring mixed:

full name:  
e-mail:  
income:  

custom nullstring for every variable:) .. you should get it by now. :P

full name:  
e-mail:  
income: