style: include RV_RETURN_VALUE_IGNORED

[vil02]

Continuation of #5122.

Note that both encrypt and decrypt call the toUpperCase().

• 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 38.27%. Comparing base (27c0978) to head (1480482).
⚠️ Report is 1081 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5152      +/-   ##
============================================
+ Coverage     38.25%   38.27%   +0.02%     
- Complexity     2361     2362       +1     
============================================
  Files           516      516              
  Lines         15311    15310       -1     
  Branches       2958     2958              
============================================
+ Hits           5857     5860       +3     
+ Misses         9166     9164       -2     
+ Partials        288      286       -2     

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