fix: IndexError exporting a zero-width dataset to reStructuredText

[santhreal]

Exporting a zero-width dataset (rows present, no columns) to rst raises IndexError. export_set guards the empty case with if not dataset.dict, but a zero-width dataset has dict == [[]] which is truthy, so it falls through to get_col(0)/column_widths[0] and indexes an empty row.

Such a dataset is valid: [[]] is valid JSON that imports cleanly (import_set sets .dict directly), and every other text format degrades gracefully (csv/tsv give a blank line, json gives [[]], html/latex give an empty table). Only rst crashes. This also returns '' when the dataset has no columns.

import tablib
tablib.import_set('[[]]', format='json').export('rst')  # IndexError before, '' after

[hugovk]

You've opened many PRs across these repos two weeks ago, and then closed them with no comment:

python-humanize/humanize#352
python-humanize/humanize#353
prettytable/prettytable#483
prettytable/prettytable#484
#659
#660
#661

Please could you explain why?

Also hundreds in other repos: