Fix "null" golang string encoded as yaml null

[denieryd]

This PR fixing this open issue: 413

Solution: incorporate each keyword from reservedNullKeywords into reservedEncKeywordMap to indicate that null keywords should be enclosed in quotes. This ensures that only Golang's nil is represented as YAML null.

Currently, any string from reservedNullKeywords is being encoded as YAML null, which is not the intended behavior and may cause confusion. Because users use Golang's nil if they want YAML null and for some reason "null" string if they want string in yaml document

This fix reverts the some behavior that was erroneously changed in this this commit. Prior to this commit, only Golang's nil was represented as YAML null.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.79%. Comparing base (3a90d71) to head (2dfbce5).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #473      +/-   ##
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+ Coverage   78.78%   78.79%   +0.01%     
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  Files          13       13              
  Lines        4007     4009       +2     
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+ Hits         3157     3159       +2     
  Misses        591      591              
  Partials      259      259