I think a lot of beggars do it not just for the money, but because they like interacting with people. So it's good to acknowledge them. Nothing wrong with offering some food if it's done in a way that gives the person some good interaction. And, anyone could get tired of eating in the same places, so it can be a treat.

But money should absolutely never be given. My technique, instead of lying and saying "I don't have anything," is to simply say "I can't help you."