[Bug] Proxy gRPC queryAssignment can set a null master broker

### Problem

`RouteActivity#queryAssignment` assumes every broker map contains the master broker id:

```java
Broker broker = brokerIdMap.get(MixAll.MASTER_ID);
...
.setBroker(broker)
```

If route metadata only has non-master broker entries, `broker` is null and the protobuf builder can fail while building the assignment.

### Expected behavior

A queue row without a master broker should be skipped. If no assignment can be built after skipping invalid rows, the existing `FORBIDDEN` / `no readable queue` response should be returned instead of failing the request exceptionally.

### Impact

Malformed or partial route metadata can make Proxy gRPC `QueryAssignment` fail instead of returning a controlled response. This affects the Proxy/gRPC client assignment path.

### Suggested fix

- Add a null check after `brokerIdMap.get(MixAll.MASTER_ID)`.
- Continue to the next queue row when the master broker is absent.
- Add regression coverage for a broker map that contains only a non-master broker id.

Start at RouteActivity#queryAssignment and trace the Proxy gRPC QueryAssignment assignment-building path. Add regression coverage for a broker map containing only a non-master broker id; done means that row is skipped and an otherwise empty result returns the existing FORBIDDEN / no readable queue response.

Written by the indexing model from the issue text.