fix(converters): preserve literal NA strings in XLSXToDocument (#12945)

[HarshitR2004]

Related Issues

• fixes XLSXToDocument drops literal NA and N/A cell values #12945

Proposed Changes:

• Default keep_default_na=False and na_values=[""] in XLSXToDocument._extract_tables().
• Prevents pandas.read_excel from coercing literal string cells like "NA", "N/A", "NaN", or "null" into missing values and converting them to empty cells.
• Preserves genuinely empty cells as empty while maintaining caller overrides via read_excel_kwargs.
• Updated read_excel_kwargs docstring to reflect the default behavior.

How did you test it?

• Added test_run_preserves_literal_na_strings: verifies literal "NA" and "N/A" are preserved in CSV and Markdown tables while empty cells remain blank.
• Added test_run_keep_default_na_override: verifies passing read_excel_kwargs={"keep_default_na": True} allows opting back into pandas' default NA parsing.
• Ran tests: hatch -e test run pytest test/components/converters/test_xlsx_to_document.py (32 passed).
• Ran type checks (mypy) and linting (ruff).

Notes for the reviewer

• Defaulting na_values=[""] alongside keep_default_na=False ensures unpopulated cells from openpyxl are still identified as missing values by pandas, so Markdown missingval and CSV formatting remain consistent.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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Hi @HarshitR2004, thanks for your interest in contributing to Haystack! 🙏

⚠️ Issue #12945 is already being addressed by open pull request(s) #12951. Before opening a PR for an issue, please check whether a PR is already linked to it, and consider contributing to the existing PR instead. We may close duplicate PRs to keep the review queue manageable.

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