[Bug] BrokerData.selectBrokerAddr crashes for an empty address table

### Before Creating the Bug Report

- [x] I found a bug, not just a question.
- [x] I searched open GitHub Issues and pull requests and found no duplicate.
- [x] I confirmed that this bug belongs to Apache RocketMQ.

### Runtime platform environment

macOS (Darwin), reproduced with an isolated local unit test.

### RocketMQ version

Branch: develop
Git commit: fd0c95920e0deac96ce2ae27442747cc5e65e930

### JDK Version

Zulu OpenJDK 8 (8.94.0.17)

### Describe the Bug

BrokerData.selectBrokerAddr assumes brokerAddrs is non-null and non-empty. A default BrokerData instance throws NullPointerException, while an empty map reaches Random.nextInt(0) and throws IllegalArgumentException.

### Steps to Reproduce

1. Create a default BrokerData and call selectBrokerAddr.
2. Alternatively, set brokerAddrs to an empty HashMap and call selectBrokerAddr.
3. Observe the exception.

### What Did You Expect to See?

The method should return null when no broker address is registered, consistent with its nullable selection result.

### What Did You See Instead?

The method throws for both null and empty address tables.

### Additional Context

Master preference and slave fallback remain valid; only the no-address boundary is affected.

Start at BrokerData.selectBrokerAddr and reproduce the issue with a default BrokerData and an empty brokerAddrs map using an isolated local unit test. Add regression coverage for both no-address cases, while confirming master preference and slave fallback remain unchanged. Done means no registered address returns null instead of throwing.

Written by the indexing model from the issue text.