style: enable AvoidNestedBlocks in checkstyle

[samuelfac]

Relates to #5119

• 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 39.25%. Comparing base (87b17e0) to head (53b7c2b).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5228      +/-   ##
============================================
+ Coverage     39.01%   39.25%   +0.24%     
- Complexity     2389     2402      +13     
============================================
  Files           517      517              
  Lines         15541    15522      -19     
  Branches       2958     2958              
============================================
+ Hits           6063     6093      +30     
+ Misses         9190     9138      -52     
- Partials        288      291       +3     

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