For attribute source rules, the current value is in the “oldValue” parameter and you can return that if you don’t want to make changes. You can instead return null and that may or may not do what you want depending on the current attribute source.
For attribute target rules, the current value may be found via the “link” parameter, and you can return that value if you don’t want to make any changes.
In a Beanshell script, not returning anything is the same as returning null.