rest: do not store empty measure set
rest: do not store empty measure set

If the measure set is empty, there's no need to store and process it
later. Just do nothing.

Change-Id: I52f84bc259485c47344fba985d6cc1cd82e599ae
Related-Bug: #1511656

gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi,leandroreox/gnocchi,sileht/gnocchi