Refactor and enhance the 'Upper' class for better functionality and robustness

[paren-thesis]

• Added proper error handling to throw for null inputs.

• Added additional string utility methods.

• 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 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.77%. Comparing base (d102fa7) to head (823fd2d).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/strings/Upper.java	66.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6118      +/-   ##
============================================
+ Coverage     73.54%   73.77%   +0.23%     
- Complexity     5108     5119      +11     
============================================
  Files           658      658              
  Lines         17649    17635      -14     
  Branches       3396     3392       -4     
============================================
+ Hits          12980    13011      +31     
+ Misses         4166     4119      -47     
- Partials        503      505       +2     

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