fix build_node_selector default value in config
@@ -28,7 +28,7 @@ c.BinderHub.use_registry = get_config('binder.use-registry', True)
c.BinderHub.per_repo_quota = get_config('binder.per-repo-quota', 0)
c.BinderHub.builder_image_spec = get_config('binder.repo2docker-image')
-c.BinderHub.build_node_selector = get_config('binder.build-node-selector', None)
+c.BinderHub.build_node_selector = get_config('binder.build-node-selector', {})
c.BinderHub.hub_url = get_config('binder.hub-url')
c.BinderHub.hub_api_token = os.environ['JUPYTERHUB_API_TOKEN']
