When is nextAction of PaymentResolve nil?

[hsatac_smartpay]

Hi,

From the API doc, it states that 

The nextAction will either be nil or contain two fields. In the case where it is nil, no next action is expected of the payments app.

I would like to know under what circumstances the nextAction is nil.

We're developing a payment gateway app, 99% of the cases we redirect user back to shopify and shopify shows them a success page.

But what should we do if the nextAction is nil? I would like to know more about this situation so we could determine what's the best user experience here.

[hsatac_smartpay]

Update:

I found that if you perform the same `PaymentSessionResolve` then only the first response contains the `nextAction` field.

This behavior seems to be changed. I can workaround with this.