docs: comments for write null fields

[NguyenHoangSon96]

Closes #

Proposed Changes

• Add comments to warn users If they write null values.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.17%. Comparing base (1213091) to head (07c6881).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage   75.17%   75.17%           
=======================================
  Files          35       35           
  Lines        2417     2417           
=======================================
  Hits         1817     1817           
  Misses        600      600           

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