Fixes the failure of updating or deleting image empty property

When property protection feature enabled, end user couldn't uses
Glance v2 API to update or delete a property of an image successfully
which value is empty, client will receive a http 500 error and a
relevant error log could be found in glance-api service log.

Closes-bug: #1332103

Change-Id: I1f9f181cea714e6ba26388d125bb7023e7a14305
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Fixes the failure of updating or deleting image empty property"