The DOMString type uses an XPCVoidableString class that inherits from
nsAutoString and adds the ability to store a JS null value. So, a boolean
comparison of a null DOMString with the null JS value yields TRUE. When the null
DOMString is passed into the JS print() method, it prints out "null". We need
to replicate this behavior for ACStrings and AUTF8Strings.
Not important right now.
Do we still want to do this? I believe we have more voidable strings now?