style: use getOrDefault in MajorityElement

[vil02]

Further simplification of the logic. Should be done in #5454.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.04%. Comparing base (7bde152) to head (a49996f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5455      +/-   ##
============================================
- Coverage     52.06%   52.04%   -0.03%     
+ Complexity     3239     3238       -1     
============================================
  Files           524      524              
  Lines         15179    15178       -1     
  Branches       2887     2887              
============================================
- Hits           7903     7899       -4     
- Misses         6954     6955       +1     
- Partials        322      324       +2     

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