[Minor] Add Nullable annotation to HttpHeaders in BrokerRequestHandler

[Jackie-Jiang]

Address #12745

[jackjlli]

LGTM. Thanks for adding the annotations!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (59551e4) to head (f2bfb82).
Report is 224 commits behind head on master.

Files	Patch %	Lines
...r/requesthandler/BrokerRequestHandlerDelegate.java	0.00%	5 Missing ⚠️
...roker/requesthandler/BaseBrokerRequestHandler.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #12816       +/-   ##
=============================================
- Coverage     61.75%    0.00%   -61.76%     
=============================================
  Files          2436     2387       -49     
  Lines        133233   131078     -2155     
  Branches      20636    20262      -374     
=============================================
- Hits          82274        0    -82274     
- Misses        44911   131078    +86167     
+ Partials       6048        0     -6048     

Flag	Coverage Δ
custom-integration1	?
integration	0.00% <0.00%> (-0.01%)	⬇️
integration1	?
integration2	0.00% <0.00%> (ø)
java-11	?
java-21	0.00% <0.00%> (-61.63%)	⬇️
skip-bytebuffers-false	0.00% <0.00%> (-61.75%)	⬇️
skip-bytebuffers-true	?
temurin	0.00% <0.00%> (-61.76%)	⬇️
unittests	?
unittests1	?
unittests2	?

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