### Elasticsearch Version

8.1.1

### Installed Plugins

_No response_

### Java Version

_bundled_

###  OS Version

Darwin Kernel Version 21.4.0

### Problem Description

Under certain conditions, executing a range enrich policy on a field with wrong datatype gives a NPE instead of a more comprehensible error.

### Steps to Reproduce

```
PUT enrich_index
{
  "mappings": {
    "properties": {
      "object": {
        "properties": {
          "field": {
            "type": "keyword"
          }
        }
      }
    }
  }
}

PUT /_enrich/policy/enrich_policy
{
  "range": {
    "indices": "enrich_index",
    "match_field": "object",
    "enrich_fields": ["enrich_field"]
  }
}

POST /_enrich/policy/enrich_policy/_execute
```

Executing the policy returns:
```json
{
  "error" : {
    "root_cause" : [
      {
        "type" : "null_pointer_exception",
        "reason" : "Cannot invoke \"String.hashCode()\" because \"<local7>\" is null"
      }
    ],
    "type" : "null_pointer_exception",
    "reason" : "Cannot invoke \"String.hashCode()\" because \"<local7>\" is null"
  },
  "status" : 500
}
```

Interestingly enough, if the enrich index were created like this (without subfield in `object`):
```
PUT enrich_index
{
  "mappings": {
    "properties": {
      "object": {
        "properties": {
        }
      }
    }
  }
}
```
 

Executing the policy would return a much better error:
```json
{
  "error" : {
    "root_cause" : [
      {
        "type" : "exception",
        "reason" : "Field 'object' has type [object] which doesn't appear to be a range type"
      }
    ],
    "type" : "exception",
    "reason" : "Field 'object' has type [object] which doesn't appear to be a range type"
  },
  "status" : 500
}
```



### Logs (if relevant)

_No response_
x-pack/plugin/enrich/src/main/java/org/elasticsearch/xpack/enrich/EnrichPolicyRunner.java
"Force merging index [{}] attempted [{}] times but did not result in one segment.",
                                    destinationIndexName,
                                    attempt,
                                    maxForceMergeAttempts
                                )
                            );
                        } else {
                            logger.debug(
                                "Policy [{}]: Force merge result contains more than one segment [{}], retrying (attempt {}/{})",
                                policyName,
                                primarySegments.getSegments().size(),
                                nextAttempt,
                                maxForceMergeAttempts
                            );
                            forceMergeEnrichIndex(destinationIndexName, nextAttempt);
                        }
                    } else {
                        
                        setIndexReadOnly(destinationIndexName);
                    }
                }
            });
    }
    private void setIndexReadOnly(final String destinationIndexName) {
        logger.debug("Policy [{}]: Setting new enrich index [{}] to be read only", policyName, destinationIndexName);
        UpdateSettingsRequest request = new UpdateSettingsRequest(destinationIndexName).setPreserveExisting(true)
            .settings(Settings.builder().put("index.auto_expand_replicas", "0-all").put("index.blocks.write", "true"));
        enrichOriginClient().admin()
            .indices()
            .updateSettings(request, listener.delegateFailure((l, r) -> waitForIndexGreen(destinationIndexName)));