In my opinion a more informative comparison would be between NULL and empty string in the same field, e.g.middle name of George Washington. Empty string means "no middle name" (or a data error if there actually is one). NULL means that nothing is said about whether there is a middle name or not and what it's value might be if there is one.

That's the general idea but of course nothing stops people from giving their own interpretation to what NULL means in a concrete situation. But that would be a modification of the original NULL concept.