NEXUS-5836: Fixing ITs

These two IT were implicitly relying on the
fact that MI's AbstractTreeNode was transferred
over the wire. It was sent with field children
being null, but once deserialized, the object's
getter getChildren() kicked in, and was never
returning null (it was lazily creating the
list if field was null).

Before NEXUS-5836, the DTO's children field was never
null when transferred over the wire (this was
confusing the UI). After NEXUS-5836 fixed,
ITs were NPEing as the getChildren() returned
what came over the wire (lazy logic gone).

After the fix, this is not the case, and introduced
DTO does not have this lazy logic anymore.