style: use getOrDefault in MajorityElement

[vil02]

This PR simplifies the logic inside the majority method.

The background story is that I was experimenting with infer and it was reporting a potential Null dereference (there are about 29 left). In this case it will not happen, but the code can be written better (as in this PR). I am considering including infer into our CI. I will keep you updated.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 52.03%. Comparing base (2f9f448) to head (33e6d27).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5454      +/-   ##
============================================
- Coverage     52.03%   52.03%   -0.01%     
+ Complexity     3238     3237       -1     
============================================
  Files           524      524              
  Lines         15181    15179       -2     
  Branches       2888     2887       -1     
============================================
- Hits           7900     7898       -2     
  Misses         6956     6956              
  Partials        325      325              

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