Add tests Generic Heap and add check null item

[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

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.04%. Comparing base (3b2ba48) to head (a21f257).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5801      +/-   ##
============================================
+ Coverage     68.80%   69.04%   +0.24%     
- Complexity     4643     4662      +19     
============================================
  Files           622      622              
  Lines         17146    17148       +2     
  Branches       3308     3308              
============================================
+ Hits          11797    11840      +43     
+ Misses         4895     4855      -40     
+ Partials        454      453       -1     

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