Always says 0 (Zero)
At present, we use a single "." in cells without any other data. We will continue using this for the present.
Numpy developers are currently considering a method to represent this situation so we will await their solution AKA Mark's NA mask at http://mail.scipy.org/pipermail/numpy-discussion/2011-August/058103.html for release with Numpy 1.7 / 2.0.
AFAICT they use "NA" where we use "." so some preliminary work may be possible.
Fixed in commit 52.