networkTests: assertBondDoesntExist with nics=None

When assertBondDoesntExist is used with nics=None, it raises an
error "'NoneType' object is not iterable".

I changed it to assert nics only when defined.

Change-Id: I43da6b2b6d7cfb79a64c37600477a69831acbf94
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31334
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>