Return empty results in group by when input is empty

[forestfang-stripe]

fixes #927

We currently have a fallback check on empty query.groups so that we can populate aggregate function default value in a non-groupby SELECT. i.e. SELECT COUNT(1) as c FROM ? WHERE 1 = 2 should return [{c:0}].

However, query.groups can also be empty if there is no input data to group by. In this case, we would expect AlaSQL (and other SQL engines indeed) to not return anything instead of emitting an undefined group. Add an additional check to verify that allgroups are empty, that is, there were no GROUP BY clause.

[mathiasrw]

Thank you!