If the distinction between "empty string" and "value is missing" (not necessarily unknown!) is important to the business, then the front end designers better make sure that there is some (preferably intuitive) way to make that distinction when entering data.

If it's not important, then the database developer should permit only one case (either by a NOT NULL constraint, or by a CHECK (column <> '') constraint, depending on what the reason is for leaving the column empty and what the expected behaviour is.