Inconsistent output with `openstack usage show`

[xiexs]

Following odd output will be displayed by the OSC command "openstack usage show", if there is no resource usage info returned.

For instance:
1) no resource usage during in a certain time range
 $ openstack usage show --project admin --start 1970-1-1 --end 1970-1-2
Usage from 1970-01-01 to 1970-01-02 on project 8f0ef608bf5848fdbbaf39297045b401:
server_usages

2)no resource usage for a certain project
$ openstack usage show --project demo
Usage from 2015-10-05 to 2015-11-03 on project 287f424504be4909970cb06d07f08a92:
server_usages

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/240761

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/240761
Committed: https://git.openstack.org/cgit/openstack/python-openstackclient/commit/?id=9e507523215bb38502832ceccea0a9b94b6f4cbd
Submitter: Jenkins
Branch: master

commit 9e507523215bb38502832ceccea0a9b94b6f4cbd
Author: xiexs <email address hidden>
Date: Mon Nov 2 04:28:08 2015 -0500

    Fix the bug of "openstack usage show"

    When there is no resouce usage associated with the project,
    an odd output will be displayed.
    This patch tried to fix this issue.

    Change-Id: I6f254c6ba37fbb760ada08e640c4938668d560dc
    Closes-Bug: #1512220