Address nillable Paddle Billing values in subscription

[oliwoodsuk]

Pull Request

Summary:
As listed here, Paddle subscription attributes next_billed_at, paused_at and scheduled_change can all be nil.
I ran into this issue when having Paddle set up in sandbox mode.

Description:
Pay::Subscription.trial_ends_at and pause_starts_at will fallback to nil is not present in the Paddle subscription.
Pay::Subscription.ends_at will default to the current date if not present, so the canceled? method works atleast.