[Bug] MessageQueuePenalizer throws when a priority group is empty

### Before Creating the Bug Report

- [x] I have searched the existing issues and pull requests.

### Runtime platform environment

N/A

### RocketMQ version

develop branch

### JDK Version

N/A

### Describe the Bug

`MessageQueuePenalizer.selectLeastPenaltyWithPriority()` delegates each priority bucket to `selectLeastPenalty()` and immediately dereferences the returned pair.

`selectLeastPenalty()` explicitly returns `null` when the queue list is null or empty. Therefore, if `queuesWithPriority` contains an empty bucket before or between non-empty buckets, `selectLeastPenaltyWithPriority()` throws `NullPointerException` instead of skipping that bucket or returning a stable empty result.

### Steps to Reproduce

1. Call `selectLeastPenaltyWithPriority()` with more than one priority group.
2. Include an empty priority group, for example `[[], [queue-0]]`.
3. Observe that the method dereferences `queueAndPenalty.getRight()` even though `queueAndPenalty` is null.

### What Did You Expect to See?

Empty priority groups should be ignored. If all priority groups are empty, the method should return `null`, matching `selectLeastPenalty()` behavior.

### What Did You See Instead?

`NullPointerException` can be thrown when a priority bucket is empty.

### Additional Context

This is a small runtime robustness issue in Proxy route queue selection.

Start at MessageQueuePenalizer.selectLeastPenaltyWithPriority() and follow its calls to selectLeastPenalty(), especially the handling of the returned pair. Reproduce the case with an empty priority group before or between non-empty groups, then verify that empty groups are ignored and that all-empty input returns null without throwing.

Written by the indexing model from the issue text.