[ISSUE #10931] Initialize cluster transaction address mapping

[Aias00]

Summary

Initialize the cluster transaction address-to-broker mapping with an empty map so a transaction sent before the first heartbeat scan does not dereference a null map.

Validation

• mvn -pl proxy -Dtest=ClusterTransactionServiceTest test

Closes #10931

[RockteMQ-AI]

Summary

LGTM. This is a clean defensive fix that prevents NPE when accessing the broker address map before the first heartbeat scan.

Key change:

• Initialize brokerAddrNameMapRef with Collections.emptyMap() instead of null
• Thread-safe immutable empty map is appropriate for AtomicReference

Test coverage:

• New test testAddTransactionDataByBrokerAddrBeforeFirstHeartbeat validates the edge case
• Verifies that accessing the map before heartbeat scan returns null gracefully instead of throwing NPE

Minimal, focused fix with proper validation.

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Automated review by github-manager-bot

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.52%. Comparing base (293f588) to head (856d25d).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #10932      +/-   ##
=============================================
- Coverage      48.62%   48.52%   -0.10%     
+ Complexity     13692    13657      -35     
=============================================
  Files           1381     1381              
  Lines         101464   101465       +1     
  Branches       13187    13187              
=============================================
- Hits           49337    49240      -97     
- Misses         46142    46219      +77     
- Partials        5985     6006      +21     

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