I don't remember getting anything cool... Except I think Chrissy sent me a piece of lovely furniture once, which I liked because I was trying to get the Lovely series.
After visiting my house, Del wrote me a message saying he saw an item that go perfect with my current design. It was... a garbage can. Yeah, Del, goes with my furniture, thanks.
I suspect they do register if you haven't written anything. Just write something generic like "how are you?" or whatever, if you don't feel like writing something new every time. Or did you mean empty as in no gift attached? Then yeah, that will have an effect too. It's not certain that they will send you anything, even if you sent them something, either.