AggregateUnionTransposeRule should account for changes in nullability of pushed down aggregates

This bug affects AggregateUnionTransposeRule.

When pushing down aggregate calls through unions, since the nullability of the aggregated columns may differ from one union branch to another, the expected nullability of the children aggregates in each union branch may differ from the original aggregates.

When assertions are enabled, this does provoke a failure when the new aggregate relations do check whether the declared type and the expected type are the same.