core: prevent SyncLunsInfo for DirectLUN

'ISCSIStorageHelper -> runConnectionStorageToDomain()' is used also
for connecting DirectLUN disks (RunVmCommandBase -> connectLunDisks),
hence storgaeDomain can be null.

Change-Id: Ifd76bb5ecaac2c71f0daefd5ddbefa2e3536599d
Signed-off-by: Daniel Erez <derez@redhat.com>