Fix NPE in DataTreeCandidates

Fix NPE in DataTreeCandidates

Patch https://git.opendaylight.org/gerrit/#/c/19473 introduced a
guaranteed NPE in DataTreeCandidates.NodeIterator. applyToModification
constructs a NodeIterator passing in a null parent. However the
NodeIterator ctor has PreCondition for non-null parent. Need to remove
the PreCondition.

(need unit tests for this code)

Change-Id: If778aaeede03a593ad2c6acf859f3741fdf0eedd
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>