From OPC UA Part 4 it was always invalid to have DataValue with a NULL Variant value with a Good StatusCode.

We recently added a dedicated Bad_NoValue StatusCode for the case where no value is provided 0007307 e.g. when loading variables from a nodeset that does not provide a value.

The AllowNulls property implies that a NULL Variant is a valid value for any DataType as long as the server has AllowNulls set to True.

As discussed in the working group F2F meeting today, this makes only sense for null-able DataTypes. The null-able DataTypes are defined in OPC UA Part 6.
But even then, (except for BaseDataType) the null value would be handled as the null-able DataType e.g. for a String DataType the server would accept a null String, not a null Variant.

Null Variant would only be valid for BaseDataType.