fix InvoiceGateway syncWithStripe() method throwing exception when the entity didn't had any upcoming invoice items
fix InvoiceGateway syncWithStripe() method throwing exception when the entity didn't had any upcoming invoice items

Signed-off-by: Bruno Gaspar <367518169777ee9ae71cfcee56304a59ac5423c1@gmail.com>

cartalyst/stripe,stevebauman/stripe,sdeering/stripe