No, it behaves like (?=\n?\z). Yours would match any embedded newline.

Oops, I was describing the situation for when /m (or (?m:)) is in effect, which was not the case.

Update - Hm, even if that were the case, it would be wrong. It would behave like (?=\n\z?|\z).

U28geW91IGNhbiBhbGwgcm90MTMgY
W5kIHBhY2soKS4gQnV0IGRvIHlvdS
ByZWNvZ25pc2UgQmFzZTY0IHdoZW4
geW91IHNlZSBpdD8gIC0tIEp1ZXJk