You can use the fn:exists XPath function, and the fn:nilled XPath function to define conditional expressions that determine whether an input element is present, or is null.
You can define conditional expressions in a transform to determine whether the transform should be applied or not in your message map.
For example, you might decide not to apply a transform if the input value is null.
Take the following steps to define a conditional expression in a Move transform that determines whether the transform is applied or not. The transform applies if the element exists, and is not null.