fix(table): preserve null extension-array values in JSON (#10299)

[hsusul]

This pull request was authored by a coding agent.

📝 Summary

Closes #10299

Problem

When a pandas DataFrame column contains an extension array (such as pint-pandas) with missing values (pd.NA, None, np.nan), PandasTableManager._to_json_str calls _extension_column_needs_stringify(result[col]) and unconditionally performs result[col] = result[col].astype(str). This stringifies missing cells into literal text like "<NA> meter" (or "nan") instead of JSON null.

Fix

Convert valid non-missing cells in extension columns to string (astype(str)), while using .astype(object).where(series.notna(), None) so missing entries remain None and serialize to JSON null as expected.

📋 Pre-Review Checklist

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✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

Testing

• uv run --group test pytest tests/_plugins/ui/_impl/tables/test_pandas_table.py — Passed (122 passed).
• uv run ruff check marimo/ tests/ — Passed with 0 errors.
• uv run ruff format --check marimo/ tests/ — Passed with 0 issues.

[hsusul]

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Project	Deployment	Actions	Updated (UTC)
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