Fix return type inference for aggregate calls: it should consider number of items in GROUP BY clause.

Alter min/max return type inference to ARG0_NULLABLE_IF_EMPTY (nullable when no keys in GROUP BY).

Fix PushAggregateThroughUnionRule.

Rework PullConstantsThroughAggregatesRule in terms of ProjectRel instead of CalcRel.

Log final plan in Prepare.

Make emp.mgr nullable (just to increase test coverage).