### 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
"Enrich policy execution for [{}] failed. No mapping available on source [{}] included in [{}]",
                policyName,
                sourceIndexName,
                policy.getIndices()
            );
        }
        return indexMapping.sourceAsMap();
    }
    private void validateMappings(final GetIndexResponse getIndexResponse) {
        String[] sourceIndices = getIndexResponse.getIndices();
        logger.debug("Policy [{}]: Validating [{}] source mappings", policyName, sourceIndices);
        for (String sourceIndex : sourceIndices) {
            Map<String, Object> mapping = getMappings(getIndexResponse, sourceIndex);
            validateMappings(policyName, policy, sourceIndex, mapping);
        }
    }
    static void validateMappings(
        final String policyName,
        final EnrichPolicy policy,
        final String sourceIndex,
        final Map<String, Object> mapping
    ) {
        
        if (mapping.get("properties") == null) {
            throw new ElasticsearchException(
                "Enrich policy execution for [{}] failed. Could not read mapping for source [{}] included by pattern [{}]",
                policyName,
                sourceIndex,
                policy.getIndices()
            );