Add tests mergesortedarray and add check list null

[lorduke22]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.78%. Comparing base (90d20b3) to head (6ee17ad). Report is 136 commits behind head on master.

Files with missing lines	Patch %	Lines
...hms/datastructures/lists/MergeSortedArrayList.java	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5809      +/-   ##
============================================
+ Coverage     59.71%   65.78%   +6.07%     
- Complexity     3855     4366     +511     
============================================
  Files           561      599      +38     
  Lines         16017    16746     +729     
  Branches       3073     3212     +139     
============================================
+ Hits           9565    11017    +1452     
+ Misses         6047     5283     -764     
- Partials        405      446      +41     

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