Read service: the ServerTimestamp should be returned when asked for non-Value attributes
The server returns null value for ServerTimestamp when non-Value attribute is read, it should returns a valid server timestamp in this case.
The specification part4 v1.03 §7.7 only indicates timestamp shall be null for source timestamp for non-Value attributes:
The sourceTimestamp is only returned with a Value Attribute. For all other Attributes the returned sourceTimestamp is set to null.