[multistage][hotfix] init execution stats map to avoid NPE

[walterddr]

regression #11746.

when early termination signal occur, it will grab the execution stats at the time, which can be null.

[codecov-commenter]

Codecov Report

Merging #11801 (b169d92) into master (0174c71) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master   #11801      +/-   ##
============================================
+ Coverage     62.99%   63.13%   +0.14%     
  Complexity     1140     1140              
============================================
  Files          2343     2343              
  Lines        126305   126306       +1     
  Branches      19419    19419              
============================================
+ Hits          79563    79742     +179     
+ Misses        41071    40896     -175     
+ Partials       5671     5668       -3     

Flag	Coverage Δ
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (ø)
integration2	0.00% <0.00%> (ø)
java-11	63.07% <100.00%> (+48.66%)	⬆️
java-17	62.98% <100.00%> (+0.01%)	⬆️
java-20	62.99% <100.00%> (+0.01%)	⬆️
temurin	63.13% <100.00%> (+0.14%)	⬆️
unittests	63.12% <100.00%> (+0.14%)	⬆️
unittests1	67.28% <100.00%> (+0.18%)	⬆️
unittests2	14.41% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...e/operator/LeafStageTransferableBlockOperator.java	86.85% <100.00%> (+0.07%)	⬆️

... and 18 files with indirect coverage changes

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