Review uses of undefined in core APIs and prefer null to represent explicit "no value".

I think our APIs would be cleaner if we used `null` wherever we have a situation where a value is missing and reserved `undefined` for cases where values have not yet be initialized.

The issue provides no file, test, or entry-point names. Begin by locating the core API uses of undefined and classify each occurrence against the proposed null-versus-uninitialized distinction; completion would require the affected uses to follow an agreed policy.

Written by the indexing model from the issue text.