[bugfix] Fixed bug in `from_networkx` (#603)

* minor fix

* small fix

python/dgl/graph_index.py

@@ -952,7 +952,11 @@ def from_networkx(nx_graph, readonly):
     num_nodes = nx_graph.number_of_nodes()
 
     # nx_graph.edges(data=True) returns src, dst, attr_dict
+    if nx_graph.number_of_edges() > 0:
         has_edge_id = 'id' in next(iter(nx_graph.edges(data=True)))[-1]
+    else:
+        has_edge_id = False
+
     if has_edge_id:
         num_edges = nx_graph.number_of_edges()
         src = np.zeros((num_edges,), dtype=np.int64)