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.

[philwebb]

We discussed this today and we'd like to keep symmetry between the fields and the getter/setters. So if the field is @Nullable the getters/setters are @Nullable. If the field is not @Nullable then the getters/setters also have no @Nullable.