Update link optiq-mat-plugin -> mat-calcite-plugin
Fix Linq4j.product; the cartesian product of 0 attributes is one row of 0 attributes.
Better handling of null values due to GROUPING SETS.

Expand GROUPING SETS at validate time, into AggregatingSelectScope, so that we know which columns become nullable, and use the same information at SQL-to-rel time.

Fix an infinite loop when validating a call in a GROUP BY clause.