schema: allow a <rule> element with no subelements in a nwfilter

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>