_concat fails if the second parameter is empty

Summary: Specifically, it produces an error "The parameters of _concat() must all be the same type; this contains both Thing Type and Unknown Type".

The issue is straightforward: _concat() looks at the type of the first parameter and requires that all subsequent params be the same type. But an empty value is often Unknown Type.

This is easy to fix, but also interesting to consider. Conceptually, Unknown Type is our version of Nothing, which means that this is more or less exactly the equivalent of concatenating Nil onto a List. This works in Scala because List is covariant, and Nothing is Bottom -- a subtype of all types. Can/should we introduce this sort of variance concept into Querki, without introducing the brain-breakingness it causes in Scala? How does variance relate to the concept of coercion that we already have?