tests: fix dry run handling in network firewall test

tests: fix dry run handling in network firewall test

The networkxml2firewalltest sets virCommand to dry run mode but doesn't
provide a callback to fill in stdout/stderr. As a result when the
firewall code queries rules it gets a NULL output and so never triggers
the callback to process output.

This trivial change just returns an empty string for the command output
in order to ensure the callback gets triggered. It has no effect right
now, but in future patches this will trigger greater test coverage.

Reviewed-by: Laine Stump <laine@laine.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>