Fix save of topology size

Fix https://github.com/GNS3/gns3-gui/issues/1561

tests/controller/test_topology.py

@@ -37,6 +37,8 @@ def test_project_to_topology_empty(tmpdir):
         "auto_start": False,
         "auto_close": True,
         "auto_open": False,
+        "scene_width": 2000,
+        "scene_height": 1000,
         "revision": 5,
         "topology": {
             "nodes": [],