Allowed create_graph to return graph with no edges
Allowed create_graph to return graph with no edges

Added a conditional in create_graph that requires a graph to have edges before calling add_edge_lengths. This allows a graph to be returned even if the overpass response includes only nodes. Otherwise, an error is thrown when add_edge_lengths is called on an edgeless graph.
gboeing/osmnx,gboeing/osmnx