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) If the initial input in webMethods is nothing (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) If nothing is typed as input in the middle, (如果在中途输入nothing), leave it blank, it will considered as NOT null, it will be evaluated as ”.
Originally from my post.