fix(csv): warn when empty input produces empty markdown

[languor-daiduode]

Fixes #1784.

Summary

• Emit a UserWarning when CSV/TSV parsing yields no meaningful rows.
• Preserve the existing empty Markdown result for compatibility.
• Add regression coverage for empty and whitespace-only input.
• Make the existing long blank-input case explicitly assert the warning.

Why

Empty CSV input previously produced an empty Markdown document without any diagnostic, making an unintentionally empty source indistinguishable from a successful conversion.

Verification

• uv run --extra all --with pytest pytest tests/test_csv.py -q: 41 passed
• uv run --with black black --check src/markitdown/converters/_csv_converter.py tests/test_csv.py: passed

[languor-daiduode]

@microsoft-github-policy-service agree