core: handle non existent OVF quota id

The fix in Ibcd0b29861a43bcb77acd7c3e0bae58d7b27b8f4 treats empty
quota, but not handling other un-existing quota(s) (i.e. quota that
was assigned to the snapshot, and then was deleted from the setup).

Leaving the former change as well since it can prevent db queries for
empty UUID (quota id is set to null).

Change-Id: I5de8f2c0cbc12405c34234958a61f61654f2d62d
Bug-Url: https://bugzilla.redhat.com/1025773
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>