What matters is the amount of space it takes up. What matters is the content of the string in the context of parsing it and displaying it. And in all of these scenarios -- the only relevant scenarios for using a string -- a null string and an empty string should almost always have identical behavior.
That's really up to the context of the problem, and the specific semantics of a null value. (Which is a value, and a present one at that. If you need to account for missing data, you can do that any number of ways, with sentinel values of any kind.)
