Substrait: Plans with AggregateRels as a last node fail to be consumed

[tokoko]

Describe the bug

First reported at ibis-substrait.

Substrait plans that have AggregateRel as the last node (and also contain both grouping expressions and measures, not just measures) fail to be consumed. The likely cause is this line in the consumer. It incorrectly assumes that number of fields in the output schema and number of measures should be the same, but that is obviously not true for the case when there are also grouping expressions in the Rel.

To Reproduce

No response

Expected behavior

No response

Additional context

No response