[ISSUE #9670]Simplify messageStore sendMessageBackHook assignment by removing null check

[mxsm]

Which Issue(s) This PR Fixes

Fixes #9670

Brief Description

How Did You Test This Change?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.01%. Comparing base (2f89371) to head (8062dae).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #9671      +/-   ##
=============================================
- Coverage      48.14%   48.01%   -0.13%     
+ Complexity     12084    12054      -30     
=============================================
  Files           1313     1313              
  Lines          92839    92838       -1     
  Branches       11868    11867       -1     
=============================================
- Hits           44694    44577     -117     
- Misses         42638    42741     +103     
- Partials        5507     5520      +13     

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[mxsm]

@RongtongJin cc