Print at least one dash for empty unaligned table columns

[gaetanzanella]

Summary

MarkupFormatter (behind Markup.format()) prints an empty delimiter cell (||) for any table column that has no alignment and no content. But an empty delimiter cell is invalid GFM — every delimiter cell must contain at least one -.

Dependencies

None.

Testing

The fix is covered by a new unit test; no manual setup is required.

Steps:

1. The new test testEmptyUnalignedColumnPrintsValidDelimiter

Checklist

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary — no public API or documented behavior changed, so no docs update needed.