fix: add null input validation to AlternativeStringArrange.arrange()

[Shubh2-0]

What

Adds explicit null-input validation to AlternativeStringArrange.arrange().

Previously, calling arrange(null, "abc") (or with the second argument null, or both null) threw a NullPointerException from inside the method with no useful message. After this change, it throws an IllegalArgumentException with the message "Input strings must not be null", matching the fail-fast convention already used by other utility classes in this package (for example HammingDistance.calculateHammingDistance).

Why

• Clearer contract: @throws IllegalArgumentException is now documented in the Javadoc, so callers know what to expect.
• Better error message: an NPE with no message is hard to debug. "Input strings must not be null" makes the cause obvious.
• Consistency: aligns with the pattern in HammingDistance and other string utilities in this package.
• No behavior change for valid inputs: all existing test cases continue to pass unchanged.

Changes

1. src/main/java/.../AlternativeStringArrange.java

   • Added a null guard at the top of arrange() throwing IllegalArgumentException.
   • Updated the method Javadoc with @throws and a note that inputs must be non-null.

2. src/test/java/.../AlternativeStringArrangeTest.java

   • Added a parameterized test arrangeThrowsOnNullInput covering all three null combinations: (null, "abc"), ("abc", null), (null, null).
   • Verifies both the exception type and the message.

Testing

All existing parameterized cases still pass. Three new null-input cases now also pass.

Checklist

• My code follows the code style of this project.
• My code does not cause any duplication.
• All new and existing tests pass.
• All new code is covered by unit tests.
• No new warnings are generated.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.63%. Comparing base (8848ed1) to head (e8bc28f).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7425   +/-   ##
=========================================
  Coverage     79.62%   79.63%           
- Complexity     7237     7241    +4     
=========================================
  Files           800      800           
  Lines         23606    23608    +2     
  Branches       4646     4646           
=========================================
+ Hits          18797    18800    +3     
- Misses         4054     4055    +1     
+ Partials        755      753    -2     

☔ 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.