fix(substrait): avoid using empty GroupingSet when groupings is empty

When groupings is empty, we should set group_exprs to the empty vector
instead of a vector containing a single empty GroupingSet.

The issue with the latter is that it triggers the addition of the extra
"__grouping_id" column in the Aggregate node, which is redundant in this
case AND causes conflicts in the output schema because Substrait does
not expect it.

Author: vbarua

datafusion/substrait/src/logical_plan/consumer/rel/aggregate_rel.rs

@@ -40,6 +40,7 @@ pub async fn from_aggregate_rel(
         let mut aggr_exprs = vec![];
 
         match agg.groupings.len() {
+            0 => {}
             1 => {
                 group_exprs.extend_from_slice(
                     &from_substrait_grouping(