refactor: NonRepeatingElement

[alxkm]

Improving javadocs.
Improving the code to make it more readable.
Adding additional tests to this algorithm.

• 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 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.91%. Comparing base (a7cd97d) to head (82ded49).

Files	Patch %	Lines
...a/com/thealgorithms/maths/NonRepeatingElement.java	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5375      +/-   ##
============================================
+ Coverage     46.79%   46.91%   +0.11%     
- Complexity     2899     2904       +5     
============================================
  Files           525      525              
  Lines         15373    15363      -10     
  Branches       2910     2908       -2     
============================================
+ Hits           7194     7207      +13     
+ Misses         7878     7855      -23     
  Partials        301      301              

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