Generate a empty Aggregate after RelFieldTrimmer#trim

[JiaTao Tao]

Hi fellows
I found this in my plan, after RelFieldTrimmer#trim,
`relBuilder.aggregate(groupKey,
newAggCallList);`, the groupKey and newAggCallList are all empty, I think
it doesn't meet expectations.

LogicalAggregate(group=[{}])
  LogicalAggregate(group=[{0, 1, 2}])


Regards!

Aron Tao