I remember from my databases course at university that NULL means that the database doesn't contain that data, and empty string means that it is known to be empty.
Let's also all be reminded about how Oracle DB doesn't let you insert empty strings, and instead treats them as NULLS even if you gave it an empty string initially.
That was a fun bug to find out, after having dealt with quite a few other DBs over the years. It was one of those "No, but surely" and "This can't be! This is Oracle!" moments. Found it while porting some old code that needed to store an empty string as being distinct from a NULL in that same column.
That's your professor's opinion, and probably one that does not come from industry experience. Look in 4 different databases and you'll see 9 different conventions. A common one is to have all strings non-null with a default value of empty string. And not all columns are strings; there is no "obviously empty" integer or boolean.
