[SLO] API fails when grouping by instanceId

[kdelemme]

Summary

Create one SLO with group by on host.name using the admin console data. When grouping by instanceId, the API call fails with this error:

http://localhost:5601/kibana/internal/observability/slos/_groups?page=1&perPage=25&groupBy=slo.instanceId&&filters=%7B%22must%22%3A%5B%5D%2C%22filter%22%3A%5B%5D%2C%22should%22%3A%5B%5D%2C%22must_not%22%3A%5B%5D%7D
{
  "statusCode": 500,
  "error": "Internal Server Error",
  "message": "Cannot convert undefined or null to object",
  "attributes": {
    "data": {}
  }
}

This is present on main, probably on 8.18 as well.