Reject null BitonicSort input explicitly

[cragkhit]

Hi. We are researchers from Mahidol University, Thailand, and the State University of Ceará, Brazil, working on a research project for improving open-source projects by using the latest accepted answer from Stack Overflow that matched your code snippet. We found this recommendation for improving your code from https://stackoverflow.com/a/64124615.

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Proposed change

Added a descriptive null-input validation and a focused regression test.

Testing

mvn -q -Dtest=BitonicSortTest test

Result: Passed

Issue: #7549

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.43%. Comparing base (1f4e2f8) to head (b6ec3c6).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7550   +/-   ##
=========================================
  Coverage     80.43%   80.43%           
- Complexity     7456     7460    +4     
=========================================
  Files           815      815           
  Lines         24053    24055    +2     
  Branches       4731     4732    +1     
=========================================
+ Hits          19346    19348    +2     
- Misses         3944     3945    +1     
+ Partials        763      762    -1     

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[DenizAltunkapan]

@cragkhit thanks!