Optimization: check_img_metadata_properties_quota should return immediately if metadata is empty

[David Jia]

When metadata is empty, the function check_img_metadata_properties_quota doesn't return correctly as expected, because the incorrect code line:
"if metadata is None:
        return"

[David Jia (jiataotj)]

althoug current code lines doesn't cause serious problem, we should correct it to avoid useless program running to benefit overall performance, so we should use "if not metadata" to replace the current code line.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/44823

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/44823
Committed: http://github.com/openstack/nova/commit/800e26f3ffd23f3f6bd5a1c09c5964d8da17ba41
Submitter: Jenkins
Branch: master

commit 800e26f3ffd23f3f6bd5a1c09c5964d8da17ba41
Author: jiataotj <email address hidden>
Date: Tue Sep 3 17:13:45 2013 +0800

    Replace incorrect Null checking to return correctly

    Null checking for metadata in check_img_metadata_properties_
    quota function doesn't work as designed, fix incorrect Null
    checking code.

    Change-Id: Id3f971091d857d6b2aebc4985cb932417c21b5dc
    Fixes: bug #1220120