Get `DuckType` from `UnionAll` without insane internals

[mrufsvold]

https://github.com/mrufsvold/DuckDispatch.jl/blob/86c957e32f55872914f97e868b3d0db2e42ca636/src/Types.jl#L51C1-L57C4

I am descending deep into the properties of UnionAll to extract a DuckType when I need it.