Well, if it came off a passionvine, it is definitely passionfruit. I can't say though why it is hollow unless what was inside dried up as the fruit became dried out. The fruit is usually picked when it becomes dark purple and a little wrinkled -- that is, if it is a Passiflora Edulis vine. It is possible for other vines to produce fruit though if cross-pollinated or self-pollinated, but there are some sterile hybrids as well.

Many hybrid passiflora, even though they may produce fruit will not produce seeds or viable seeds. I have seen those same hollow fruits with P. Constance Elliot, for example.

Cocomo