TraceTreeItemOwner: Change behavior for empty containers

An empty TraceTreeItemOwner's v_extents are no longer
[INT_MAX, INT_MIN] but [0, 0] instead. As new items were added
relative to the view's own v_extents, items were added at
positions close to the over-/underflow margin. This messed up
the scrollbar among other things. Returning [0, 0] instead
allows for better handling of such cases.