Clarify handling of null/empty values

[benno]

Should the following all be equivalent?

{
  "given": "Pat",
  "family": "Lee"
}

{
  "given": "Pat",
  "middle": "",
  "family": "Lee"
}

{
  "given": "Pat",
  "middle": null,
  "family": "Lee"
}

If not, what should the behavior be?

Either way, update the documentation to clarify.