Unit members of unions have invalid TypedNode rendering

[kubukoz]

To reproduce, generate and compile the code for:

$version: "2"

namespace demo

@trait
union aaa {
    u: Unit
    s: String
}

@aaa(u: {

})
string Hello

The error:

[error] ./demo/Hello.scala:13:5
[error] object UCase in object Aaa does not take parameters
[error]     demo.Aaa.UCase(()).widen,
[error]     ^^^^^^^^^^^^^^

The actual signature of UCase is case object UCase, indeed with no parameters.

This has been discovered in the latest spec for SNS, in one of the smoke test traits.