[Multi-stage] Fix SortedMailboxReceiveOperator to not pull 2 EOS blocks

[Jackie-Jiang]

Currently SortedMailboxReceiveOperator will always pull 2 EOS blocks from underlying mailboxes, which doesn't follow the convention. This PR fixes it by caching the first EOS block so that it can be returned in the next getNextBlock()

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (04b279e) 61.69% compared to head (eee5737) 61.72%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12406      +/-   ##
============================================
+ Coverage     61.69%   61.72%   +0.03%     
  Complexity      207      207              
============================================
  Files          2428     2428              
  Lines        132823   132823              
  Branches      20544    20544              
============================================
+ Hits          81948    81989      +41     
+ Misses        44859    44822      -37     
+ Partials       6016     6012       -4     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (ø)
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (ø)
integration2	0.00% <0.00%> (ø)
java-11	61.65% <100.00%> (-0.01%)	⬇️
java-21	61.60% <100.00%> (+26.74%)	⬆️
skip-bytebuffers-false	61.71% <100.00%> (+0.02%)	⬆️
skip-bytebuffers-true	61.55% <100.00%> (+26.71%)	⬆️
temurin	61.72% <100.00%> (+0.03%)	⬆️
unittests	61.72% <100.00%> (+0.03%)	⬆️
unittests1	46.91% <100.00%> (+0.03%)	⬆️
unittests2	27.71% <0.00%> (+<0.01%)	⬆️

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