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

Attention: 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).

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 

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.