Aggregation with collectSet on empty dataset produces Null

Consider the following example:

import frameless.functions.aggregate.{collectSet, max, min}
import frameless.syntax._
import frameless.TypedDataset

case class Foo(bar: Int)
val ds = TypedDataset.create(List.empty[Foo])

ds
  .agg(
    min(ds('bar)),
    collectSet(ds('bar))
  )
  .collect
  .run

It produces WrappedArray(null) while the expected value would be WrappedArray() because the initial dataset is empty.
Please note that this bug happen only when collectSet is combined with an other aggregation. Aggregating only with min produces the expected result.

While using only collectSet

ds.agg(collectSet(ds('bar))).collect.run

Produces WrappedArray(Vector()) while the expected value is WrappedArray()

I haven't tested with the other collect functions llke collectList

Start by reproducing the empty TypedDataset example with collectSet combined with min, then inspect the collectSet aggregation implementation and related tests. Done means empty input returns WrappedArray() both alone and when combined with another aggregation, without changing the behavior of non-empty inputs.