The problem is precisely that NULL is not some sort of Maybe monad, but people keep trying to use it as such. It's a lot like using NaN as a sentinel value for floats - sure, you can do that, but when something goes wrong, instead of an error at the point where the problem is, you end up dealing with a mysterious NULL somewhere way down the line. And that's the best case - the worst is that you get wrong query results because of the way NULL comparisons work.

An empty string is better as a sentinel value because at least this doesn't have the weird "unknown value" semantics that NULL does. But if you really want the same level of explicitness and safety as an option type, the theoretically proper way to do this in relational model is to put the strings themselves in a separate table in a 1:N (where N is 0 or 1) relationship with the primary table.