Table component bugs — falsy values lost, runtime crash when columns missing, class typo, and unstable keys

This PR fixes multiple issues in `next-cloudinary/docs/components/Table/Table.js`:

- Preserve falsy values (like `0` and empty string) in cells by only replacing `null`/`undefined`.
- Add default fallbacks for `columns` and `data` to avoid runtime crashes.
- Guard usage of `columns` in tbody.
- Fix Tailwind class typo `last pr-0` -> `last:pr-0`.
- Avoid variable shadowing by renaming column index variable to `colIndex`.
- Use stable row keys (`row.id ?? rowIndex`) instead of just rowIndex.

Testing:
- Render table with `data` containing `0` and ensure `0` displays.
- Render component with `columns` undefined and some `data` and ensure no runtime error.
- Verify header / cell spacing respects `last:pr-0`.