refactor: OctalToDecimal

[alxkm]

Improving readability.
Improving tests.

• 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

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 43.44%. Comparing base (2905ccb) to head (e22a49b).
Report is 2 commits behind head on master.

Files	Patch %	Lines
.../com/thealgorithms/conversions/OctalToDecimal.java	80.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5344      +/-   ##
============================================
+ Coverage     43.38%   43.44%   +0.06%     
- Complexity     2690     2693       +3     
============================================
  Files           522      522              
  Lines         15448    15446       -2     
  Branches       2953     2954       +1     
============================================
+ Hits           6702     6711       +9     
+ Misses         8459     8447      -12     
- Partials        287      288       +1     

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[alxkm]

PMD warning can't be fixed.

Warning: PMD Failure: com.thealgorithms.conversions.OctalToDecimalTest:21 Rule:UnnecessaryFullyQualifiedName Priority:4 Unnecessary qualifier 'Assertions': 'assertEquals' is already in scope.

Tried different approaches to fix it, but the warning persists even those files does not changed.