add null handling to sketch group-by

[walterddr]

for some reason this was hit when sketch loaded/used directly from binary

e.g.

SELECT 
  groupKey,
  distinctCountThetaSketch(rawSketchBinaryCol) -- this can some times cause group key internal null value 
FROM tbl
GROUP BY 1

so we need the null handling code path in extractGroupByResult similar to extractAggregateResult

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (2e367a2) 61.55% compared to head (70e40d1) 61.56%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
...n/DistinctCountThetaSketchAggregationFunction.java	0.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #12259   +/-   ##
=========================================
  Coverage     61.55%   61.56%           
+ Complexity     1153     1152    -1     
=========================================
  Files          2416     2416           
  Lines        131205   131211    +6     
  Branches      20250    20252    +2     
=========================================
+ Hits          80768    80781   +13     
+ Misses        44539    44534    -5     
+ Partials       5898     5896    -2     

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	61.51% <0.00%> (-0.01%)	⬇️
java-21	61.43% <0.00%> (-0.01%)	⬇️
skip-bytebuffers-false	61.53% <0.00%> (-0.01%)	⬇️
skip-bytebuffers-true	61.41% <0.00%> (-0.01%)	⬇️
temurin	61.56% <0.00%> (+<0.01%)	⬆️
unittests	61.56% <0.00%> (+<0.01%)	⬆️
unittests1	46.61% <0.00%> (+<0.01%)	⬆️
unittests2	27.74% <0.00%> (+<0.01%)	⬆️

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