Follow-up from "Add variants": witness: `null`

The following discussion from !269 (merged) should be addressed:

• @mgoebel started a discussion: (+1 comment)

  the referenced demo shows witnesses referenced with null:

  {
  "witness": null,
  "entry": "omisit"
  },
  {
  "witness": null,
  "entry": "ܚܪܫܐ"
  }