Allow GET_UNSEEN_NOTIFICATIONS_BADGE undefined payload

[kydorn]

In some cases in which the native implementation does not have a current value for the unseen notifications badge (when it has not checked the current value yet or it has not been set by other means), the apps will send the response with no payload.

We need the GET_UNSEEN_NOTIFICATIONS_BADGE method to support responses without payload.

Also,the exported method:
getUnseenNotificationsBadge: () => Promise<{unseenNotificationCounter: number; lastUpdated: number}>;
would need to change to something like:
getUnseenNotificationsBadge: () => Promise<{unseenNotificationCounter: number; lastUpdated: number} | void>;