When a field in the artifact returns a null value through REST (for instance permissions on artifact field) then some keys are added into the json content:

"values": [
"0": {
"field_id": 596,
"type": "date",
"label": "Start Date",
"value": "2019-06-24T00:00:00+02:00"
},
"2": {
"field_id": 600,
"type": "sb",
"label": "Status",
"values": [
{
"id": "405",
"label": "Planned",
"color": null,
"tlp_color": null
}
],
"bind_value_ids": [
405
]
}
]

This keys can also be added into the webhook payload.