The problem is not in the JSON structure, but in the export mapping.
You should set Send empty values to false in your export mapping.
On what is the export mapping based? JSON or XML? Because you could adjust the XSD or JSON. Based on that information the modeler decides if the attribute is optional or nillable.
Regards,
Ronald
[EDIT]
If optional is false it means that the attribute should always be in the message. Nillable means that the value could be empty. So in your case occurance should be 1 and nillable set to true. If occurance is not one double check the JSON to see if every attribute is always present even if the value is empty.