Do not send subscription emails when nothing is due or active

[novoselt]

https://sagemathcloud.zendesk.com/agent/tickets/14395 may be relevant to track details for those who have access.

It seems that when automatic payments are enabled, there is a "warning" email that there will be a payment processed in a week. This user used to have a subscription, but it was cancelled, so nothing was due and nothing was expected to become due. Either there is no check for "zero expected balance" or it is working incorrectly.

[williamstein]

I think this is caused by stripe and there's no solution at this point in time from our point of view, except "don't use stripe checkout". I've added upstream though, since maybe stripe will (or did) fix this.