NEXUS-5836: Browse Index Tree fix

When pulling out the MI TreeView from direct
use in REST API (as DTO), during conversion
the resulting payload was slightly different
and that made UI think it deals with leafs, and
never asked for siblings.

Problem actually lies in UI, that neglects the
leaf boolean flag, but it actually tested
for children field (was null, after change
it was empty list, now is again null). Ultimately
it's the leaf flag that should be used to test is a
node a leaf or not, not the value of children array
(UI assumes if children == null, is not leaf,
otherwise it is).

The REST resource did work, and even IT did pass
as the IT uses the node:leaf and node:chilren
properties as should
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.
NEXUS-5836: Applied comments

* local assignment of children
* import order in IT
NEXUS-5836: Import order, no code change
Merge pull request #96 from sonatype/nexus-5836-browse-index

NEXUS-5836: Browse Index Tree fix