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