suite: do not calculate product on an empty list (take 2)

The original attempt to fix this with dda7954090ccc9ffd8ecd961d6b530a0e6d24964
failed because I screwed up the indentation for the out.append(...) line.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/teuthology