core: AuditLogable - replace check to isNullOrEmpty

AuditLogable - getVmIdRef:
replace nullity check with Guid.isNullOrEmpty
as mVmId member is pre-set to Guid.Empty.
The change is needed for cases when just the
VM object is set (rather than the VmId)

Change-Id: I894592ecac51674c708c1e9aba8716e4971493cb
Signed-off-by: Daniel Erez <derez@redhat.com>