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     

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