Empty array should be null in raw-json

[JayHuLBL]

Current raw-json output have the sections like:

"if_expression": {
       "if_elseif": []
}

It should be null so not in the json output. The cause is in the file ExpressionVisitor.java that the if_elseif array does not become null even if it is empty.

This issue is to make the change in the java files that may have the similar issue.