handle data frames with 0 columns in `rel_from_df()`

[romainfrancois]

related to https://github.com/hannes/duckdb-rfuns/pull/91#discussion_r1592100105

[hannes]

Thanks, seems like we have to revisit the question of whether we want to allow zero-column result sets, as I recall last time we discussed this we decided not to have those but circumstances may have changed.

[Tmonster]

Found this document I wrote when we first encountered the question.

https://docs.google.com/document/d/1oKaCP62b8tjkOooX7rNUVGwF3waM3PhBMzuB-PLxqtc/edit?usp=sharing

It seems like for aggregations with no groups, results are inconsistent between DuckDB/sqlite and dplyr

[krlmlr]

Thanks, Tom. Aggregation with no groups is different, this is about data frames without columns.