use a simple way to determine whether manifest is null

[Hua Wang]

wsme.Unset has override __nozero__ function, so we can use a simple way to determine whether a wsme type is null.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/205022

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/205022
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=8405305b0d429f59a3e963fea3f2aaa5fe9f6714
Submitter: Jenkins
Branch: master

commit 8405305b0d429f59a3e963fea3f2aaa5fe9f6714
Author: Hua Wang <email address hidden>
Date: Thu Jul 16 08:14:32 2015 +0800

    Use a simple way to determine whether a wsme type is null

    wsme.Unset has override __nozero__ function, so we can use a simple
    way to determine whether a wsme type is null.

    Change-Id: I77c38ea811a58524a25873d156d53578246a4976
    Closes-Bug: #1477524