### 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
assertThat(
                thrown.getMessage(),
                containsString(
                    "Enrich policy execution for ["
                        + policyName
                        + "] failed while validating field mappings for index ["
                        + sourceIndex
                        + "]"
                )
            );
            assertThat(
                thrown.getCause().getMessage(),
                containsString(
                    "Could not traverse mapping to field [nesting.key]. The [nesting" + "] field must be regular object but was [nested]."
                )
            );
        } else {
            fail("Expected exception but nothing was thrown");
        }
    }
    public void testRunnerValueNestedSourceMapping() throws Exception {
        final String sourceIndex = "source-index";
        XContentBuilder mappingBuilder = JsonXContent.contentBuilder();
        mappingBuilder.startObject()
            .startObject(MapperService.SINGLE_MAPPING_NAME)
            .startObject("properties")
            .startObject("key")
            .field("type", "keyword")
            .endObject()
            .startObject("nesting")