[Bug] Cluster transaction service throws before broker address mapping is initialized

## Problem

`ClusterTransactionService` initializes `brokerAddrNameMapRef` without a map. Before the first successful transaction-heartbeat scan, `getBrokerNameByAddr` dereferences `brokerAddrNameMapRef.get()` directly.

`AbstractTransactionService.addTransactionDataByBrokerAddr` calls this method on the transaction send path, so an otherwise valid transaction can fail with `NullPointerException` during proxy startup or before route discovery has completed.

## Expected behavior

An unresolved broker address should follow the existing `AbstractTransactionService.addTransactionDataByBrokerName` behavior: return `null` and do not record transaction data.

## Proposed fix

Initialize the address-to-broker-name mapping with an empty map (or null-check it) and add a regression test covering `addTransactionDataByBrokerAddr` before the first heartbeat scan.

## Scope

Proxy cluster transaction service. This does not change transaction protocol behavior after route discovery.

Start with ClusterTransactionService.getBrokerNameByAddr and AbstractTransactionService.addTransactionDataByBrokerAddr, then compare the existing addTransactionDataByBrokerName behavior. Add the regression test for calling the address-based path before the first transaction-heartbeat scan; done means an unresolved address returns null without recording transaction data and no NullPointerException occurs.

Written by the indexing model from the issue text.