List of all nulls (Nothing? type) produces unexpected ColumnGroup column kind instead of ValueColumn

[koperagen]

Minimal repro:

fun main() {
    val df2 = dataFrameOf(
        "nulls" to List(3) { null }.toColumn(),
    )
}

Expected:
one value column
nulls: Nothing?

Actual:
Nothing? gets picked up by this.isSubtypeOf(typeOf<AnyRow?>()) -> ColumnKind.Group branch

internal fun KType.toColumnKind(): ColumnKind =
    when {
        this.isSubtypeOf(typeOf<AnyFrame>()) -> ColumnKind.Frame
        this.isSubtypeOf(typeOf<AnyRow?>()) -> ColumnKind.Group
        else -> ColumnKind.Value
    }

This leads to strange side effects like this pretty normal looking code failing with exception:

val df2 = dataFrameOf(
    "nulls" to List(3) { null }.toColumn(),
    "lists" to List(3) { n -> List(n) { n } }.toColumn()
)
df2.explode { lists }

Exception in thread "main" org.jetbrains.kotlinx.dataframe.exceptions.UnequalColumnSizesException: Unequal column sizes. Expected rows count: 0. Actual column sizes:
nulls: 0
lists: 3

Fix:
Just add a Nothing? -> Kind.Value branch