Manage empty graph

This can happen if the edgelist file is not found.

Overlap/report_graph.py

@@ -143,15 +143,16 @@ if __name__ == "__main__":
     n1 = len_H
     n2 = len_H
 
-    # First and last dates:
-    node_index = min(G.nodes)
-    my_date = node_to_date_eddy(node_index, G.graph["e_overestim"],
-                                only_date = True)
-    print("First date:", my_date)
-    node_index = max(G.nodes)
-    my_date = node_to_date_eddy(node_index, G.graph["e_overestim"],
-                                only_date = True)
-    print("Last date:", my_date)
+    if len_H >= 1:
+        # First and last dates:
+        node_index = min(G.nodes)
+        my_date = node_to_date_eddy(node_index, G.graph["e_overestim"],
+                                    only_date = True)
+        print("First date:", my_date)
+        node_index = max(G.nodes)
+        my_date = node_to_date_eddy(node_index, G.graph["e_overestim"],
+                                    only_date = True)
+        print("Last date:", my_date)
 
     if args.merging:
         print("\nMerging:")