Can a heap have no elements?
It's up to you to decide. It's a pathological case which is quite similar to the empty graph (on zero vertices) &mdash; sometimes it makes sense to admit it, and sometimes not. It depends on the circumstances. The definition in Wikipedia is irrelevant here &mdash; there are many other similar definitions in the literature, that might disagree on this point.

I recommend the classic article Is the null-graph a pointless concept by Harary and Read.