Refactor assertString: Faster, less nested and more consistent.

[EmersonRabelo]

@profnandaa, I have a possible improvement for the assertString. I am reducing the number of necessary validations and also a more assertive approach, in my point of view (if this approach doesn't make sense, I would appreciate feedback on it, if possible). The performance has also improved, it wasn't something incredible, but it was considerable.

Unit tests:

[codecov[bot]]

Codecov Report

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

Comparison is base (b958bd7) 99.95% compared to head (f8e62de) 99.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2372      +/-   ##
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- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files         107      107              
  Lines        2454     2445       -9     
  Branches      619      617       -2     
==========================================
- Hits         2453     2444       -9     
  Partials        1        1              

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[EmersonRabelo]

Hi, @WikiRik! I made a small change in the code and also committed the unit tests. But it failed in one of the tests.

[WikiRik]

codecov/project will be fixed with https://github.com/validatorjs/validator.js/pull/2341

[pano9000]

when you pass literally null, it gives

Uncaught TypeError: Cannot read properties of null (reading 'constructor')

because it is not caught by the first check.

Should we explicitly check for null there as well?

[EmersonRabelo]

Hello, @pano9000! Do you have any further comments on this or is everything okay? If necessary, I will make the alterations!