[Transform] transform fails with "specified fields can't be null or empty"

[hendrikmuhs]

found by @szabosteve

Affected versions: 7.9.3 - 7.11.0

The transform fails calling _preview or on _start with the error message: specified fields can't be null or empty. This happens for transforms that don't use any field parameter in either group_by or aggregation, e.g. because in group_by/aggregation only scripts are used or a filter aggregation without a child aggregation (for a full example see below).

Mitigation:

Create the destination index, before starting the transform yourself, e.g. PUT myindex.

or

Add an aggregation that uses a field into your config, e.g:

    "aggregations": {
      "count": {
        "value_count": {
          "field": "my_grouping_field"
        }
      },

Solution:

The bug is a regression of #62945. In a special case transform tries to retrieve field capabilities for 0 fields, which is not allowed.

full failure example:

POST _transform/_preview
{
  "source": {
    "index": [
      "kibana_sample_data_logs"
    ]
  },
  "pivot": {
    "group_by": {
      "agent": {
        "terms": {
          "script": {
            "source": """String agent = doc['agent.keyword'].value;
            if (agent.contains("MSIE")) {
              return "internet explorer";
            } else if (agent.contains("AppleWebKit")) {
              return "safari";
            } else if (agent.contains('Firefox')) {
              return "firefox";
            } else { return agent }""",
            "lang": "painless"
          }
        }
      }
    },
    "aggregations": {
      "200": {
        "filter": {
          "term": {
            "response": "200"
          }
        }
      },
      "404": {
        "filter": {
          "term": {
            "response": "404"
          }
        }
      },
      "503": {
        "filter": {
          "term": {
            "response": "503"
          }
        }
      }
    }
  }
}

Error:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "specified fields can't be null or empty"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "specified fields can't be null or empty"
  },
  "status" : 400
}

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)