1) if it's blank space in DB (after trim service), it will be equal to '';
2) if convert empty to null, it will reach $null statement;
3) 如果在webmethods里面,在最开始直接输入的是nothing, leave it blank, it will be considered as $null; it will be evaluated as $null.
4) 如果在中途输入nothing, leave it blank, it will considered as NOT null, it will be evaluated as ''.