Calling GetItemRect() for an item which was not currently visible
because its parent was collapsed resulted in silently returning the
value for a wrong value before the recent fix to GetRowByItem() and in
a crash after it because GetTreeNodeByRow() returned null when passed
invalid row index.
Fix this by explicitly checking whether the item is shown and just
returning an empty rectangle instead.
Also document this behaviour and add a unit test for it.
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