A user-defined type the server sent as a literal, handed back as data.
Spark serialises a UDT literal as a struct whose type names a class rather than field names,
so there is nothing to key a map by. Latu.Result.Literal.value/1 returns this instead: the
class, and the elements decoded in the order that class defines them. Latu interprets none of
them; latu_ml matches on :class for the two spark.ml uses.
:class is the JVM class the type declared, and nil for a Python UDT; the elements decode
either way.