[multistage] Handle Column Less Tuples in BlockSplitter

[ankitsultana]

Small change to fix broken support for column less tuples.

This issue was caught while I was working on #15958.

Not adding new tests for this since it will be covered by #15958.

Verified the fix with Colocated Join Quickstart:

SET useMultistageEngine=true;
SET usePhysicalOptimizer=true;

select COUNT(*) OVER() from userAttributes

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.39%. Comparing base (1a476de) to head (9a0d7bb).
Report is 225 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #16058      +/-   ##
============================================
+ Coverage     62.90%   63.39%   +0.49%     
+ Complexity     1386     1355      -31     
============================================
  Files          2867     2913      +46     
  Lines        163354   167138    +3784     
  Branches      24952    25553     +601     
============================================
+ Hits         102755   105960    +3205     
- Misses        52847    53155     +308     
- Partials       7752     8023     +271     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.33% <100.00%> (+0.46%)	⬆️
java-21	63.33% <100.00%> (+0.51%)	⬆️
skip-bytebuffers-false	?
skip-bytebuffers-true	?
temurin	63.39% <100.00%> (+0.49%)	⬆️
unittests	63.39% <100.00%> (+0.49%)	⬆️
unittests1	56.50% <100.00%> (+0.68%)	⬆️
unittests2	33.44% <0.00%> (-0.14%)	⬇️

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