cache: fix sending of empty

[Davide D'Amico]

After 0.8.3[678], I didn't see any row in changelog about the problem in
subject.
Have I still to apply patches as in
http://nginx.org/pipermail/nginx-devel/2010-January/000101.html?

It was a wrong fix in 0.8.31, so why it's still there?

Thanks for your work.

d.