various nullable/optional properties (#37935)
various nullable/optional properties (#37935)

- for ISubscription, ISubscriptionItem, IPlan, ITier
markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped