The requirements stated in Part 3 and Part 4, which are in the Additional Information field, can be read to be conflicting. While Part 3 describes the possibility to return NULL values for nullable datatypes Part 4 states that NULL values can only be received with a bad severity.

Part 3:
This Property specifies if a null value is allowed for the Value Attribute of a DataVariable which has a nullable DataType.
Part 4:
If the DataType of the Variable is not BaseDataType, the Severity shall be BAD if the value is NULL.

There are cases where both requirements can be fulfilled like writing a NULL value by a client and the server returns a Bad severity when the value is read from clients. But this use case would ideally rather be done with a proper WriteMask and writing the StatusCode directly. For other cases it is not clear which spec applies.

Wording should be enhanced one way or the other to be clear in which cases a NULL needs to be expected by clients and what servers are allowed to provide for the different use cases.
When working on the solution also look at Arrays, Structures, Alarm Instances. Also consider use cases where an application wants to reset values because there is currently no value (Bad_NoValue is per definition only for no initial value).