Empty fruit - and the commentary extends the word to flowers, leaves and roots - is what does not satisfy hunger but destroys much to be eaten: little food, many lives.
The commentary's examples are the bael fruit, the rose-apple, the mahua flower and the drumstick: things with a great deal to throw away and a little to eat, where every part thrown away was a body. It is the plate's proportion rule, and the only item whose reason is a ratio rather than a presence.
The twenty-two, laid by their reasons - Empty fruit highlighted