I'll answer - there really isn't one. You could say "undefined" but we already have another word for that ("undefined") - an "empty" construct doesn't exist in any other programming language that I'm aware of. Back in PHP's way-too-helpful days, they came up with this concept, so they got to define what it meant. (And to be fair, as with other things from the bad old days, it has caused confusion ever since.)
If an int with 0 is empty a string with a only white spaces is the same kind of empty.
I think empty mixes the computer meaning of empty with the linguistic meaning.