[Bug] NullPointerException in AdminBrokerProcessor.updateAndCreateTopicList at line 636

### Before Creating the Bug Report

- [x] I found a bug, not just asking a question, which should be created in [GitHub Discussions](https://github.com/apache/rocketmq/discussions).

- [x] I have searched the [GitHub Issues](https://github.com/apache/rocketmq/issues) and [GitHub Discussions](https://github.com/apache/rocketmq/discussions) of this repository and believe that this is not a duplicate.

- [x] I have confirmed that this bug belongs to the current repository, not other repositories of RocketMQ.

### Runtime platform environment

OS name: "linux", version: "5.15.0-138-generic", arch: "amd64", family: "unix"

### RocketMQ version

version: rocketmq-all-5.4.0
link: https://github.com/apache/rocketmq/archive/refs/tags/rocketmq-all-5.4.0.zip

### JDK Version

Java version: 1.8.0_452
Build tool: Apache Maven 3.9.1

### Describe the Bug

A NullPointerException occurs when invoking AdminBrokerProcessor.processRequest with a RemotingCommand created via RemotingCommand.createRequestCommand(...) that has a null body. The issue arises because the method CreateTopicListRequestBody.decode(...) returns null when the input byte array is null, and the subsequent call to requestBody.getTopicConfigList() dereferences this null value.

### Steps to Reproduce

Test Code:

```java
package org.apache.rocketmq.broker.processor;
import org.apache.rocketmq.broker.processor.AdminBrokerProcessor;
import org.junit.Test;
import org.apache.rocketmq.broker.BrokerController;
import io.netty.channel.ChannelHandlerContext;
import org.apache.rocketmq.remoting.protocol.RemotingCommand;
import org.apache.rocketmq.remoting.protocol.RequestCode;
import org.apache.rocketmq.common.BrokerConfig;
import org.apache.rocketmq.common.MixAll;
import org.apache.rocketmq.remoting.netty.NettyClientConfig;
import org.apache.rocketmq.remoting.netty.NettyServerConfig;
import org.apache.rocketmq.store.config.MessageStoreConfig;

public class TestClass {
@Test(timeout=3000)
public void test() throws Throwable {
BrokerConfig brokerConfig = new BrokerConfig();
NettyServerConfig nettyServerConfig = new NettyServerConfig();
NettyClientConfig nettyClientConfig = new NettyClientConfig();
MessageStoreConfig messageStoreConfig = new MessageStoreConfig();
BrokerController brokerController = new BrokerController(brokerConfig, nettyServerConfig, nettyClientConfig, messageStoreConfig);
AdminBrokerProcessor adminBrokerProcessor0 = new AdminBrokerProcessor(brokerController);
RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.UPDATE_AND_CREATE_TOPIC_LIST, null);
adminBrokerProcessor0.processRequest((ChannelHandlerContext) null, request);
}
}
```

Execution Result:

```
JUnit version 4.13.2
.E
Time: 0.869
There was 1 failure:
1) test(org.apache.rocketmq.broker.processor.TestClass)
java.lang.NullPointerException
at org.apache.rocketmq.broker.processor.AdminBrokerProcessor.updateAndCreateTopicList(AdminBrokerProcessor.java:636)
at org.apache.rocketmq.broker.processor.AdminBrokerProcessor.processRequest(AdminBrokerProcessor.java:269)
at org.apache.rocketmq.broker.processor.TestClass.test(TestClass.java:24)

FAILURES!!!
Tests run: 1, Failures: 1
```

### What Did You Expect to See?

The method should either:

- Validate that the request body is non-null before decoding, or
- Handle the case where the decoded CreateTopicListRequestBody is null gracefully (e.g., return an error response).

### What Did You See Instead?

A NPE is thrown:

```
java.lang.NullPointerException
at org.apache.rocketmq.broker.processor.AdminBrokerProcessor.updateAndCreateTopicList(AdminBrokerProcessor.java:636)
at org.apache.rocketmq.broker.processor.AdminBrokerProcessor.processRequest(AdminBrokerProcessor.java:269)
at org.apache.rocketmq.broker.processor.TestClass.test(TestClass.java:24)
```

### Additional Context

Root Cause Analysis

1. RemotingCommand.createRequestCommand(...) does not initialize the body field.

2. In updateAndCreateTopicList, the code calls:

```java
final CreateTopicListRequestBody requestBody = CreateTopicListRequestBody.decode(request.getBody(), CreateTopicListRequestBody.class);
```

Since request.getBody() returns null, decode(...) returns null.

3. The next line unconditionally accesses:

```java
List topicConfigList = requestBody.getTopicConfigList(); // NPE here
```

Start in AdminBrokerProcessor.updateAndCreateTopicList at line 636 and inspect CreateTopicListRequestBody.decode with a null request body. Run the supplied JUnit reproduction through AdminBrokerProcessor.processRequest. Done means a request created without a body no longer throws a NullPointerException and is handled gracefully, such as by returning an error response.

Written by the indexing model from the issue text.