The null expression is used only for containing elements (elements whose group type is not None), and only when the element has the null property set in the element properties. For leaf elements (elements with a group type is None), you don't need the null expression; if the input being mapped to the element is null, then the output will be null. If you wish to have special conditions for emitting null then you can use the Constant function to emit the null or you can test for null using the IsNull function.
Similar to the generation of the Emit expression, when you map an input to an output, the containing elements are examined in order starting from the parent of the output map element and the output is matched with the container of the input map element and so on up the line. For each matching containing map element, an IsNull is generated, so that the generation of the null for the output is controlled by its corresponding input.