### Elasticsearch Version

8.2.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Cloud

### Problem Description

Creating a PIT without the required `?keep_alive` parameter throws a NPE which maps to HTTP status code `500 Internal Server Error` whereas really this isn't a server-side error, it's a bad request, so the status code should be something like `400 Bad Request`

### Steps to Reproduce

```
# Request:
POST /_pit

# Response:
# 500 Internal Server Error
# {
#   "status": 500,
#   "error": {
#     "reason": "[keep_alive] parameter must be non null",
#     "root_cause": [
#       {
#         "reason": "[keep_alive] parameter must be non null",
#         "type": "null_pointer_exception"
#       }
#     ],
#     "type": "null_pointer_exception"
#   }
# }
```

### Logs (if relevant)

_No response_
server/src/main/java/org/elasticsearch/action/search/OpenPointInTimeRequest.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.action.search;
import org.elasticsearch.action.ActionRequest;
import org.elasticsearch.action.ActionRequestValidationException;
import org.elasticsearch.action.IndicesRequest;
import org.elasticsearch.action.support.IndicesOptions;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.tasks.Task;
import org.elasticsearch.tasks.TaskId;
import java.io.IOException;
import java.util.Map;
import java.util.Objects;
import static org.elasticsearch.action.ValidateActions.addValidationError;
public final class OpenPointInTimeRequest extends ActionRequest implements IndicesRequest.Replaceable {