Fix null handling in streaming selection operator

[cbalci]

Null handling support was not added to the StreamingSelectionOnlyOperator. This is causing default values being returned for streaming/gRPC queries, even if the query option enableNullHandling=true is set.
This change fixes it.

Testing

• Added unit test coverage
• Ran manual tests to confirm gRPC endpoint returns null for null columns, rather than defaults.

bugfix

[codecov-commenter]

Codecov Report

Merging #10453 (c465623) into master (cb1069f) will increase coverage by 0.07%.
The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master   #10453      +/-   ##
============================================
+ Coverage     70.24%   70.32%   +0.07%     
+ Complexity     6110     6108       -2     
============================================
  Files          2061     2061              
  Lines        111752   111763      +11     
  Branches      16985    16991       +6     
============================================
+ Hits          78504    78595      +91     
+ Misses        27720    27638      -82     
- Partials       5528     5530       +2     

Flag	Coverage Δ
integration1	24.53% <0.00%> (+0.14%)	⬆️
integration2	24.23% <23.07%> (+<0.01%)	⬆️
unittests1	67.99% <61.53%> (+0.03%)	⬆️
unittests2	13.91% <0.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...ator/streaming/StreamingSelectionOnlyOperator.java	86.27% <76.92%> (-3.73%)	⬇️

... and 39 files with indirect coverage changes

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[Jackie-Jiang]

LGTM!