Compiler plugin schema breaks for DataFrame with Platform Type (`!`)

[Jolanrensen]

These types can be introduced from Java code. Even though nullability is not known, these types are considered to be not null across Kotlin and should thus be treated as such.

Workaround is explicitly casting to the non-null type or changing the .toDataFrame() function to return DataFrame<T & Any>, suppressing the INCORRECT_LEFT_COMPONENT_OF_INTERSECTION error.