fix: serialize Pandas NaN values into LineProtocol

[bednar]

Closes #590

Proposed Changes

Fixed serialisation Pandas NaN values into LineProtocol.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] pytest tests completes successfully
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

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

Project coverage is 90.41%. Comparing base (a645ea9) to head (1afb72e).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #648 +/- ## ========================================== - Coverage 90.42% 90.41% -0.02% ========================================== Files 39 39 Lines 3509 3505 -4 ========================================== - Hits 3173 3169 -4 Misses 336 336 ```

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