Indeed, but it tells you nothing other than that there is no amount and not how we have to construe this missing amount. In other words, the database must find meaning outside of itself and that goes against the principles of relational model.

Everyone seems to misunderstand what I mean: Yes "NULL" can be quite practical, but it goes against the basic principles of the relational model, so I try to avoid it as much as possible. It is the same with database design: there is no law that says all your databases have to be in the Fifth Normal Form, but it is really handy if they are and you will avoid a number of problems (or at least it is easier to avoid these), but is it always worth the extra effort?

So use "NULL" if you like, but know what you are doing and why.