message: Make schema compliant by fixing CloseTab* index definitions
  * The index field should be of type `"null"` and not `null`.
  * The default value should be `null` and not `""`
  Closes
diff: "index": {
  "oneOf": [
  { "type": "integer" },
  - { "type": null }
  + { "type": "null" }
  ],
  - "default": "",
  + "default": null,
  "description": "Close the tabs other than the one at this index. If no index is provided, use the focused tab's index."
  }
  }
  "index": {
  "oneOf": [
  { "type": "integer" },
  - { "type": null }
  + { "type": "null" }
  ],
  - "default": "",
  + "default": null,
  "description": "Close the tabs following the tab at this index. If no index is provided, use the focused tab's index."
  }
  }
  