(rasqal_new_rowsequence_rowsource): Allow an empty sequence of rows
GROUP BY generates 1 row with no bindings if it gets no data in

(rasqal_groupby_rowsource_process): Do not set group_iterator unless
there were inner rows.

(rasqal_groupby_rowsource_read_row): Use group_iterator set/not to
distinguish case where inner rowsource was empty.

(main): Add test for new case - new test0
agg-2 group by with no data expected result is 1 row with no bindings