fix(xlsx/xls): skip completely empty sheets that emit broken tables

[yi111 (Yi-111-a)]

Summary

• Skip sheets where DataFrame.columns.empty in both XlsxConverter and XlsConverter, so a completely empty worksheet no longer emits malformed table markdown (|\n| |).
• Header-only sheets (which still have columns) continue to render as a well-formed empty table.
• Add tests for completely empty sheets, header-only sheets, and an all-empty workbook.

Fixes #2470

Test plan

• packages/markitdown/tests/test_xlsx_empty_sheet.py passes
• A workbook with one data sheet and one blank sheet no longer includes the blank sheet heading or broken table syntax
• Header-only sheets still produce a valid empty markdown table

[yi111 (Yi-111-a)]

@microsoft-github-policy-service agree