Interface writing problems

My interface is like this. when there is a returned value, the data in the json will have a value, or it will be an empty string that the colleague told me that when he received the data, data is converted to an object in json format for retrieval. if the data is empty, the entire program will pop up, or the data will return null and cannot be returned...

My interface is like this. when there is a return, the data in the json will have a value, otherwise it will be an empty string.

My colleague told me that when he received the message, the data was converted to an object in json format for retrieval. if the data was empty, the entire program would crash, or I would return null for data, cannot return null. I don't understand. Is that true?