refactor: improve Sparsity class with input validation and clearer logic

[alxkm]

• Added null and empty matrix checks to prevent invalid input
• Simplified zero counting logic for readability
• Enhanced JavaDoc for better method documentation

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.61%. Comparing base (c997a32) to head (9335386).
⚠️ Report is 377 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/misc/Sparsity.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6351      +/-   ##
============================================
- Coverage     74.61%   74.61%   -0.01%     
  Complexity     5451     5451              
============================================
  Files           682      682              
  Lines         19022    19024       +2     
  Branches       3677     3677              
============================================
  Hits          14194    14194              
- Misses         4270     4271       +1     
- Partials        558      559       +1     

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