Account for reported UNMODIFIED nodes

Account for reported UNMODIFIED nodes

DataObjectModification has no way of expressing UNMODIFIED nodes,
but they can (and are) reported from DataTreeCandidateNode, hence
we end up reporting IllegalStateException when the user tries to
look at those nodes.

Make sure we take the node under consideration and do not report
unmodified children, hence preventing the ISE from happening.

JIRA: MDSAL-422
Change-Id: I34d36ae083c5ce3ad793eb584236f175f7a3a906
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>