docs: missing values return null, not undefined

[ralacerda]

Documentation is saying that missing values are undefined, but they are in fact null.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.84%. Comparing base (4d61c78) to head (a0230f2).
Report is 74 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
- Coverage   65.05%   58.84%   -6.22%     
==========================================
  Files          39       38       -1     
  Lines        4055     3115     -940     
  Branches      487      522      +35     
==========================================
- Hits         2638     1833     -805     
+ Misses       1408     1273     -135     
  Partials        9        9              

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[pi0]

Thanks.