OPENDJ-2787 Replication: NPE at server start when re-importing a dataset

This problem is triggered by a change in behaviour of DN.valueOf(String).
In the server DN, if the string value was null, it was returning the root DN,
while the SDK DN throws a NullPointerException.

ModifyDNMasg.java:
In newParentIsEqual(), added a null check.
In newDNIsParent() and newDNIsEqual(), added a fix for the change in behaviour of DN.valueOf()
(now throws LocalizedIllegalArgumentException instead of DirectoryException).