When adding 죠 or 지요 to a sentence it is a way of comfirming that the information you have is right. Like in this case "There are none, right?". The ending is equivalent to 'right' or 'isnt it'. Also 죠 is used in speech and 지요 standard Korean. 없어요 - There are none / They don't have any 없죠 - There are none, right? / They don't have any right?