If you move a smart group to trash, the “smart” bit of it remains active; so you will still see a disclosure triangle to the left of the name of the smart group. If that is pointing down, then the content of the smart group will be disclosed - i.e. you will see everything which fulfils the conditions of the smart group. If, however, you select one of those files and ⇧⌘i you will see that its location is not “trash” but rather wherever the document resides in your database (that’s as opposed to the smart group itself, the location of which will be listed as trash). As such, emptying the trash does not delete any of the files listed in the smart group (they are not contained in the smart group).
If that is not the case (i.e. the location of the file is trash), then that would be unexpected behaviour.
(I have just tested this, and the behaviour was correct, and as expected and described in the first paragraph of my response.)