### 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
@Override
    public void run() {
        try {
            logger.info("Policy [{}]: Running enrich policy", policyName);
            task.setStatus(new ExecuteEnrichPolicyStatus(ExecuteEnrichPolicyStatus.PolicyPhases.RUNNING));
            
            final String[] sourceIndices = policy.getIndices().toArray(new String[0]);
            logger.debug("Policy [{}]: Checking source indices [{}]", policyName, sourceIndices);
            GetIndexRequest getIndexRequest = new GetIndexRequest().indices(sourceIndices);
            
            client.admin().indices().getIndex(getIndexRequest, listener.delegateFailure((l, getIndexResponse) -> {
                try {
                    validateMappings(getIndexResponse);
                    prepareAndCreateEnrichIndex(toMappings(getIndexResponse));
                } catch (Exception e) {
                    l.onFailure(e);
                }
            }));
        } catch (Exception e) {
            listener.onFailure(e);
        }
    }
    private List<Map<String, Object>> toMappings(GetIndexResponse response) {
        return response.mappings().values().stream().map(MappingMetadata::getSourceAsMap).collect(Collectors.toList());
    }
    private Map<String, Object> getMappings(final GetIndexResponse getIndexResponse, final String sourceIndexName) {
        ImmutableOpenMap<String, MappingMetadata> mappings = getIndexResponse.mappings();
        MappingMetadata indexMapping = mappings.get(sourceIndexName);
        if (indexMapping == MappingMetadata.EMPTY_MAPPINGS) {
            throw new ElasticsearchException(