test: Added to WordLadderTest corner case where wordList is empty

[jasonjyu]

Added to WordLadderTest corner case where wordList is empty

• [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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.72%. Comparing base (f3e0900) to head (a4240f0).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5908   +/-   ##
=========================================
  Coverage     68.72%   68.72%           
  Complexity     4630     4630           
=========================================
  Files           620      620           
  Lines         17112    17112           
  Branches       3302     3302           
=========================================
  Hits          11761    11761           
  Misses         4900     4900           
  Partials        451      451           

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[jasonjyu]

@yanglbme, @vil02, @BamaCharanChhandog, @alxkm, @siriak, would you have some time to review my PR? Thanks.