Exception raises when GET request is sent to batch endpoint

Was getting:
```
ValueError at /graphql/batch/
max() arg is an empty sequence
```

Author: femesq

graphene_django/views.py

@@ -105,8 +105,11 @@ def dispatch(self, request, *args, **kwargs):
 
             if self.batch:
                 responses = [self.get_response(request, entry) for entry in data]
-                result = '[{}]'.format(','.join([response[0] for response in responses]))
-                status_code = max(responses, key=lambda response: response[1])[1]
+                if not responses:
+                    result, status_code = self.get_response(request, data, show_graphiql)    
+                else:
+                    result = '[{}]'.format(','.join([response[0] for response in responses]))
+                    status_code = max(responses, key=lambda response: response[1])[1]
             else:
                 result, status_code = self.get_response(request, data, show_graphiql)