unified-iso: handle empty arch
    
    Prior to this change, when running pungi-create-unified-iso on a compose
    with zero builds present for an arch, unified-iso crashes.
    
    The problem is that unified-iso does not set up the arch's debuginfo
    destination directory at all before trying to dump the productmd
    treeinfo for that arch's debuginfo. productmd tries to write to the
    destination directory that does not exist.
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>