engine :  NPE on logging task with no vdsm task I

When rebuilding commands with no vdsm task id
an exception could result in NPE in AsyncTaskUtils

Change-Id: Iae9dbf08af4459dba05ee3f90b58e605bb3b900c
Bug-Url: https://bugzilla.redhat.com/1112555
Signed-off-by: Ravi Nori <rnori@redhat.com>