If you attempt to set the value of an attribute to NULL when using IBM ECM Content Manager (CM) API, it causes it to be overwritten by its default value, as NULL is not recognized as a valid value. This applies to all attributes, where a default value is set in the System Administration Client.
Setting an empty value is not possible via API, because the attribute is configured to have a default value.
Diagnosing the problem
When you run Java code with setNull(dataID) method called, it will result in the default value being set.
Resolving the problem
This is working as designed. CM is designed to accept empty values if and only if there is no default value defined for the attribute. As soon as there is a default value defined in System Administration Client, CM will not accept empty values for this particular attribute.