Enhance Alphabetical.isAlphabetical with null and empty string check

[Ayesha-Rathnayake]

• [ x ] I have read CONTRIBUTING.md.
• [ x ] This pull request is all my own work -- I have not plagiarized it.
• [ x ] All filenames are in PascalCase.
• [ x] 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.
• [ x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

Enhanced Alphabetical.isAlphabetical method:

• Added null and empty string check
• Introduced current and next variables for readability
• Improves safety and maintainability

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.80%. Comparing base (3eb521b) to head (ca212b4).

Files with missing lines	Patch %	Lines
...n/java/com/thealgorithms/strings/Alphabetical.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #6689 +/- ## ============================================ - Coverage 75.80% 75.80% -0.01%  + Complexity 5804 5802 -2  ============================================ Files 705 705 Lines 19846 19850 +4 Branches 3851 3850 -1 ============================================ + Hits 15045 15047 +2  Misses 4218 4218 - Partials 583 585 +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.