Better handling of unavailable shops for RecurringApplicationCharge

The current implementation will throw a `NoMethodError: undefined method 'find' for nil:NilClass` exception when calling `ShopifyAPI::RecurringApplicationCharge.current`, `ShopifyAPI::RecurringApplicationCharge.pending`, etc on an uninstalled or otherwise unavailable shop.

This improvement will simply return a `nil`, which is consistent with `ShopifyAPI::RecurringApplicationCharge.all` for the uninstalled shop