[ISSUE #9750] Fix NPE when isAsyncSendEnable is false

[RongtongJin]

Which Issue(s) This PR Fixes

Fixes #9750
Fixed NPE issue in transaction metrics recording logic when isAsyncSendEnable is false.

Brief Description

• Solution: Added sendTransactionPrepareMessage check in the condition to ensure transaction metrics are only recorded when processing transaction prepare messages
• Consistency: Aligns with the logic in asynchronous send mode (line 342 already has the same condition check)

How Did You Test This Change?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.23%. Comparing base (40ca80e) to head (57e9bc4).
⚠️ Report is 158 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ocketmq/broker/processor/SendMessageProcessor.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #9751      +/-   ##
=============================================
- Coverage      48.32%   48.23%   -0.10%     
+ Complexity     12193    12169      -24     
=============================================
  Files           1313     1313              
  Lines          93340    93340              
  Branches       11962    11962              
=============================================
- Hits           45111    45021      -90     
- Misses         42660    42734      +74     
- Partials        5569     5585      +16     

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