Available in LIB version 2016.3.1121, it will be also available in the 2017 R1 Release.
Thank you for the update. I agree, this is a core mechanism. As it is not currently implemented in the control, that's why we consider it as new functionality rather than a bug (which suggests a broken functionality).
I think that it's a bug report, not a feature request, because TargetNullValue is part of the Binding mechanism. One of the core mechanics of WPF is broken!