schema: allow a <rule> element with no subelements in a nwfilter
    
    This is a regression in behavior caused by commit 37359814. It was
    intended to limit the schema to allow only a single subelement of
    <rule>, but it is also acceptable for <rule> to have no subelement at
    all.
    
    To prevent the same error from reoccurring in the future, the
    examples/xml/nwfilter directory was added to the list of nwfilter
    schema test directories.
    
    Resolves: https://bugzilla.redhat.com/1593549
    
    Signed-off-by: Laine Stump <laine@laine.org>
    ACKed-by: Michal Privoznik <mprivozn@redhat.com>