This is intended to complement the "defaults" function as part of the
optional attributes experiment, to give module authors a way to concisely
declare that they don't care to distinugish between null objects/tuples
and objects/tuples _containing_ nulls, and that they don't care to
distinguish between null collections and empty collections, and thus they
can avoid needing to do conditional traversals and focus only on handling
the leaf primitive-typed values as being null.

This is just a prototype to see what this might look like and how it will
interact with the "defaults" function. It's not yet ready to be merged
because it lacks tests and documentation.