Unexpected warning when create image without data

[wangxiyuan]

When create an image without data via api v2.0.

It will raise a warning like:
SAWarning: The IN-predicate on "image_locations.id" was invoked with an empty sequence. This results in a contradiction, which nonetheless can be expensive to evaluate. Consider alternative strategies for improved performance.

from the code:
https://github.com/openstack/glance/blob/master/glance/db/sqlalchemy/api.py#L798

The reason is that 'location_data' 's value is [], not None. So the function '_image_locations_set' will be always executed.