Review nullability of configuration properties setters

[wilkinsona]

We have a pattern where properties that are initialised with a default value are not @Nullable and, therefore, the setter's parameter isn't @Nullable either. Where the property has no default value it is @Nullable and both the getter and setter are @Nullable. This means that whether a setter is @Nullable depends on whether or not the property has a default value. The two feel unrelated to me and I wonder if we want to be consistent in the nullability of configuration properties setters.