suite: do not calculate product on an empty list

This fixes a bug where a directory with just % generates a bogus job item
with no actual content.  (e.g.,

$ find basic basic basic/%
$ teuthology-suite --dry-run --base . --collections basic --name foo 
INFO:teuthology.suite:Collection basic in ./basic 
INFO:teuthology.suite:configs [('basic/{}', [])]

...which then blows up because the job yaml is empty.  With the fix, there
are no generated jobs:

$ teuthology-suite --dry-run --base . --collections basic --name foo 
INFO:teuthology.suite:Collection basic in ./basic 
INFO:teuthology.suite:configs []

(The configs print was temporarily added for debugging purposes.)

Signed-off-by: Sage Weil <sage@inktank.com>