### 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
}
        
        try {
            validateField(mapping, policy.getMatchField(), true);
            for (String valueFieldName : policy.getEnrichFields()) {
                validateField(mapping, valueFieldName, false);
            }
        } catch (ElasticsearchException e) {
            throw new ElasticsearchException(
                "Enrich policy execution for [{}] failed while validating field mappings for index [{}]",
                e,
                policyName,
                sourceIndex
            );
        }
    }
    private static void validateField(Map<?, ?> properties, String fieldName, boolean fieldRequired) {
        assert Strings.isEmpty(fieldName) == false : "Field name cannot be null or empty";
        String[] fieldParts = fieldName.split("\\.");
        StringBuilder parent = new StringBuilder();
        Map<?, ?> currentField = properties;
        boolean onRoot = true;
        for (String fieldPart : fieldParts) {
            
            Object type = currentField.get("type");
            if (type != null && "object".equals(type) == false) {
                throw new ElasticsearchException(
                    "Could not traverse mapping to field [{}]. The [{}] field must be regular object but was [{}].",
                    fieldName,
                    onRoot ? "root" : parent.toString(),