gluster: fix testcase failure in VolumeDao

Empty GUID will not be  assigned to taskId of Volume while
reading from database. Also return an empty AsnycTask if taskId
is not present.

Change-Id: I104f59d9dc170a1b151046e16de6ad5c35279e55
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>