finishedAt should be null until completion

finishedAt should be null, like duration.  

```json
{
  "indexUid": null,
  "status": "processing",
  "type": "dumpCreation",
  "uid": 245713,
  "batchUid": 203454,
  "canceledBy": null,
  "details": {
    "dumpUid": null
  },
  "error": null,
  "duration": null,
  "startedAt": "2026-03-09T17:05:21.538Z",
  "enqueuedAt": "2026-03-09T17:05:21.497Z",
  "finishedAt": "1970-01-01T00:00:00.000Z"
}
```