Actually... allan will have to back me up on this but I think it's impossible. An empty line has nothing on it but the ending return, and then only if it's not the last line. Therefore there is nothing to actually be matched, because the insertion point gets the scope of the char before the point, on an empty line it would be the scope of the previous lines return which you can't match because TM can only match one line at a time.
So actually I guess the answer is your match is valid... but because there is no real character to match it makes it useless in practice. At least I think... :)