Empty DataFrame with `Nothing` column creates String column when written to Arrow

[Jolanrensen]

As follow up for #1807

Empty DataFrame with Nothing column creates String column when written to Arrow/Feather (probably others too):

val original = dataFrameOf("emptyCol" to DataColumn.empty())

val featherBytes = original.saveArrowFeatherToByteArray()
val fromFeather = DataFrame.readArrowFeather(featherBytes)
fromFeather shouldBe original // fails, type is String instead of Nothing

This is likely because Nothing matches isSubtypeOf(typeOf<String?>()) in KType.toArrowField()