test: adding tests asserting None are translated to "None".

[germa89]

Description

Adding tests to make sure the conversion None to "None" is correct.

Issue linked

Close #3660

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

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[germa89]

@pyansys-ci-bot LGTM.

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.07%. Comparing base (e6221a6) to head (216a678).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3694      +/-   ##
==========================================
- Coverage   87.14%   87.07%   -0.07%     
==========================================
  Files         187      187              
  Lines       14697    14698       +1     
==========================================
- Hits        12808    12799       -9     
- Misses       1889     1899      +10