simple fix no model file (#125)

visualdl/server/visualDL

@@ -184,7 +184,10 @@ def histogram():
 @app.route('/data/plugin/graphs/graph')
 def graph():
     # TODO(ChunweiYan) need to add a config for whether have graph.
-    data = {'url': '/graphs/image'}
+    if graph_image_path is None:
+        data = {'url': ''}
+    else:
+        data = {'url': '/graphs/image'}
     result = gen_result(0, "", data)
     return Response(json.dumps(result), mimetype='application/json')