You can use an ESQL statement to compare an element to NULL.
If you want to compare an element to NULL, code the statement:
IF InputRoot.DFDL.MyMessage.Elem2.Child1 IS NULL THEN
DO:
-- more ESQL --
END IF;
If nulls are permitted for this element, this statement tests whether the element exists in the input message, or whether it exists and contains one of the 'nil' values defined for the element in the DFDL schema. The returned result of this test is determined as follows:
If you want to determine if the field is missing, rather than present but with null value, you can use the ESQL CARDINALITY function.