Fix SubscriptionExpired error when the subscription is still valid

[IrvanFza]

This PR fixes the SubscriptionExpired error while the current subscription is still active. Turn out that when the subscription is still active, the response doesn't have a cancellation_date value. So I skip the entire expiration check if cancellation_date is empty.

Active subscription response example:

BillingInfo {
    subscription: SubscriptionInfo {
        cancellation_date: None,
    },
    trial: TrialInfo {
        trial_expiration_date: Some(
            "2019-11-17T04:02:39.550054Z",
        ),
    },
}

Related discussion: #5 (comment)

[IrvanFza]

P.S.: This is the first time I read and write code in Rust, so please share your thoughts if the code is bad.