Ticket #415 - winsync doesn't sync DN valued attributes if DS DN value doesn't exist
    
    Description:
    Once we had a NULL vs checking (Ticket #415):
    commit 737169e0afd0547b6de0a61081508314d076cc6a,
    but it was accidentally removed by the next commit (Ticket 346):
    commit 890fc22687821279f3862fc0862a4e8d93c00291.
    
    This patch adds the NULL checking for vs.