### 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/test/java/org/elasticsearch/xpack/enrich/EnrichPolicyRunnerTests.java
}
    private void validateSegments(String createdEnrichIndex, int expectedDocs) {
        IndicesSegmentResponse indicesSegmentResponse = client().admin()
            .indices()
            .segments(new IndicesSegmentsRequest(createdEnrichIndex))
            .actionGet();
        IndexSegments indexSegments = indicesSegmentResponse.getIndices().get(createdEnrichIndex);
        assertNotNull(indexSegments);
        assertThat(indexSegments.getShards().size(), is(equalTo(1)));
        IndexShardSegments shardSegments = indexSegments.getShards().get(0);
        assertNotNull(shardSegments);
        assertThat(shardSegments.shards().length, is(equalTo(1)));
        ShardSegments shard = shardSegments.shards()[0];
        assertThat(shard.getSegments().size(), is(equalTo(1)));
        Segment segment = shard.getSegments().iterator().next();
        assertThat(segment.getNumDocs(), is(equalTo(expectedDocs)));
    }
    private void ensureEnrichIndexIsReadOnly(String createdEnrichIndex) {
        ElasticsearchException expected = expectThrows(
            ElasticsearchException.class,
            () -> client().index(
                new IndexRequest().index(createdEnrichIndex)
                    .id(randomAlphaOfLength(10))
                    .source(Map.of(randomAlphaOfLength(6), randomAlphaOfLength(10)))
            ).actionGet()
        );
        assertThat(expected.getMessage(), containsString("index [" + createdEnrichIndex + "] blocked by: [FORBIDDEN/8/index write (api)]"));
    }
}