DisplayTopology: fix NPE for empty topology

This fixes a bug introduced when refactoring calls of getAbsoluteBounds
to allNodesIdMap. The latter is more lightweight but does not handle
an empty topology like getAbsoluteBounds does. Return an empty map if
the topology is empty, rather then throw an NPE.

Bug: 416247314
Flag: EXEMPT bugfix
Test: SQ
Change-Id: I748dda801bb15a0546b731271dfe5567fdf14b34