### 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
IndexNameExpressionResolver resolver = getInstanceFromNode(IndexNameExpressionResolver.class);
        Task asyncTask = testTaskManager.register("enrich", "policy_execution", new TaskAwareRequest() {
            @Override
            public void setParentTask(TaskId taskId) {}
            @Override
            public TaskId getParentTask() {
                return TaskId.EMPTY_TASK_ID;
            }
            @Override
            public Task createTask(long id, String type, String action, TaskId parentTaskId, Map<String, String> headers) {
                return new ExecuteEnrichPolicyTask(id, type, action, getDescription(), parentTaskId, headers);
            }
            @Override
            public String getDescription() {
                return policyName;
            }
        });
        ExecuteEnrichPolicyTask task = ((ExecuteEnrichPolicyTask) asyncTask);
        
        
        
        ActionListener<ExecuteEnrichPolicyStatus> wrappedListener = new ActionListener<>() {
            @Override
            public void onResponse(ExecuteEnrichPolicyStatus policyExecutionResult) {
                testTaskManager.unregister(task);
                listener.onResponse(policyExecutionResult);
            }
            @Override
            public void onFailure(Exception e) {
                testTaskManager.unregister(task);