Passing a null value to any Corticon Server using XML payloads is accomplished in the following ways:

Vocabulary Type Passing a null in an XML message
An attribute of any type Omit the XML tag for the attribute, or use the XSD special value of xsi:nil='1' as the attribute's value.
An attribute except String types Include the XML tag for the attribute but do not follow it with a value, for example, <weight></weight> or simply  <weight/>. If the type is String, this form is treated as an empty string (a string of length zero, which is not the same as null).
An association Do not include an href to a potentially associable Entity (Flat model) or do not include the potentially associable role in a nested child relationship to its parent.
An entity Omit the complexType from the payload entirely.

XML Payloads with null values created by Rules

When Rules set a null value that propagates into the payload of a request, XML treats the null as follows

Assume that the incoming payload is...

      Person
        Age  : 45
        Name : Jack

... and that rule processing sets Age = null.

The output would remove Age from the payload like this: 
      Person
        Name : Jack