routing: make setting source node optional when parsing testdata

routing/pathfind_test.go

@@ -209,10 +209,12 @@ func parseTestGraph(path string) (*channeldb.ChannelGraph, func(), aliasMap, err
 		}
 	}
 
+	if source != nil {
 		// Set the selected source node
 		if err := graph.SetSourceNode(source); err != nil {
 			return nil, nil, nil, err
 		}
+	}
 
 	// With all the vertexes inserted, we can now insert the edges into the
 	// test graph.