Fallback Values for Nulls in Expressions

[Noggog]

Would be nice to have the binding setups allow for fallback functionality when an expression encounters a null. Rather than the RxUI pattern of not firing anything, this system would fire the fallback value whenever the expression chain is broken by the presence of a null.

Something like

this.WhenPropertyChanges(x => x.Property1.Property2!.MyInt, fallback: default(int));

I imagine this could either could be the default functionality, and/or only take effect if a fallback value is provided like above.