dg: fix getNode()
dg: fix getNode()

The semantics is that it will return nullptr when
the node with given key does not exist in the graph.
Previously, the function created it (used [] operator)
in this case

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg